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we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is .
is there such a thing as a negative force ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels .
and if the third law is true why ca n't we stand on water ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth .
if there was no gravity would the force exerted by table would make the box to fly upward ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens .
are there any special or different characteristics that differentiate the newton 's third law partner forces from the partner forces that are equal and opposite and are not newton 's third law partner forces ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels .
wait , when i push my hands against each other , does the effect happen because of the newton 's second or third law ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects .
how can an object exert a force on other object without touching it and there are so far from each other with a certain distance ... ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal .
what is the direction of the acceleration of the planets ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table .
is one of the forces is attractive and another is repulsive ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels .
but if the third law instantly affects , then can we have a communication method that is faster than the speed of light ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously .
then can we know the supernova happens instantly before its light reaches to the earth ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects .
why does n't the glass return the ball back ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right .
2nd question : if i hit the wall with hand and move it 1 cm why does n't the wall exert a force that pushes my hand back at least 1 cm ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law .
the acceleration is going to be the net force divided by the mass , so will there be two accelerations with one net force ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes .
how can a small planet exert that much force ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law .
how will one planet be closer to the star ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle .
mercury is closer to the sun than the earth ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth .
so the box pushes upward on the table and the table push the box downward ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force .
what does the term victor means ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects .
if not , what does the amount of force the earth exerts on each object depend on ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is .
so i get that you can not apply a force fast enough to `` avoid '' the opposite force , but what if the force you apply is done faster than the speed of light ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects .
if you were to punch through an object such as plank of wood or a brick would the object still exert the same amount of force that you hit it with ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason .
when we throw the bubble gum or sand ball on the ground it doesnt have any reaction , plz explain why ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law .
i thought a planet orbits the center of mass of the solar system not the star ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth .
in the video , if the table and the box are accelerating , what are the net force exerted on them ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object .
if it is accelerating , that does n't necessarily mean the reaction force at is greater than the gravity , right ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens .
why will two non partner forces cancel out if there is no acceleration ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth .
how table is applying the force on the box ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a .
how do we determine the direction of the force fat ( force on a by the table ) ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens .
according to newton 's third law of motion the two forces acting on each other can be equal , but the acceleration produced may not be equal.can someone explain me how will it happen ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens .
what are conservative and non-conservative forces ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens .
what are newton'third law partner forces ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever .
is there any system/situation when the third law of motion applies to two objects without touching with each other ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b .
how is f = -f ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on .
i know you 're probably talking about different things , but if according to newton 's second law , an object that is n't accelerating does not have a force ( f=ma ) , how does newton 's third law hold here ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up .
or the other way around , if the box does have a force on the earth ( equal and opposite to earth 's gravitational pull on the box ) , then why does n't the box have acceleration ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens .
if there are 3 objets exerting forces on each other like a table , book over it , and the earth , wil we still apply newton 's 3rd law ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on .
a planet and its moon ) , would there be two pairs of partner forces ( the force of gravity exterted from each object and the resulting partner force from each force of gravity ) ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces .
so assuming that the third law is correct , this surely means that there must always be 2 objects in the universe ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ?
`` you ca n't catch the universe sleeping '' well ... is n't there a speed cap at which information can travel at ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ?
but how can it be instantaneous if the speed of light is the fastest possible ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on .
if obj a and b smash into each other with equal but opposite forces , and apparently they always do , then the force applied per kg of mass , should be different for both objects , right ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a .
so if a is 100x the mass of b , then the amount of force applied on b upon a , per 1 kg on average , is 100x less than the force a applies upon b , per 1kg , right ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth .
in the example , box on a table , there is a force downward , gravitational force acting on the box then there should be another force which is equal and in opposite direction , so there is an upward force , force exerted on earth by box but how can this happen as the box was on the table there is no contact between earth and box ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector .
but what if our mass is equal ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ?
is n't there supposed to be an opposite force ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is .
why do you say the earth exerts a force on a ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object .
then if box a would float to the ice mass you ca n't say earth enacts a force on a , you say the frozen water ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth .
why not use a constant to refer to attraction by mass or gravity ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law .
if a car 's acceleration is 300 m/s^2 during a crash , what magnitude force would cause this acceleration ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick .
theoretically , if chuck norris kicked the wall at a velocity in the positive direction with a magnitude higher than the speed of light ... would the force of the wall on chuck norris ' foot still be instantaneous ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same .
if the two different object have the same mass , will the total acceleration then be zero ?
we should talk a little more about newtons 's third law , because there are some deep misconceptions that many people have about this law . it seems simple , but it 's not nearly as simple as you might think . so people often phrase it as , for every action there 's an equal and opposite reaction . but that 's just way too vague to be useful . so a version that 's a little better , says that for every force , there 's an equal and opposite force . so this is a little better . the equal sign means that these forces are equal in magnitude . and this negative sign means they 're just different by the direction of the vector . so these are vectors , so this says that this pink vector f , has the opposite direction , but equal in magnitude to this green vector f. but to show you why this is still a little bit too vague , consider this , if this is all you knew about newtons 's third law , that for every force , there 's an equal and opposite force , you might wonder , if you were clever , you might be like , wait a minute , if for every force f , right , there 's got to be a force that 's equal and opposite . well why does n't that just mean that every force in the universe cancels ? should n't every force just cancel then , at that point ? does n't that just mean that there 's no acceleration that 's even possible ? because if i go and exert a force f on something , if there 's gon na be a force negative f , does n't that mean that no matter what force i put forward , it 's just gon na get cancelled ? and the answer no , and the reason it 's no is because these two forces are exerted on different objects . so you have to be careful . so the reason i say that this statement of newtons 's third law is still a little bit too vague , is because this is really on different objects . so if this is the force on object a , exerted by object b , then this force over here has to be the force on object b , exerted by object a . in other words , these forces down here are exerted on different objects . i 'm gon na move this over to this side . i 'm gon na move this over to here . let 's draw two different objects to show explicitly what i mean . so if there was some object a , so i put some object a in here . just wan na make sure there 's an object a . let 's say this is object a , and it had this green force exerted on it , f. so this object right here is a . well , there 's gon na be another object , object b . we 'll just make it another circle . so we 'll make it look like this . so here 's object b . and it 's gon na have this pink force , f , negative f exerted on it . so i 'm gon na call this object b . now we 're okay , now we know these forces ca n't cancel , and the reason these forces ca n't cancel , is cause they 're on two different objects . but when you just say that newtons 's third law , is that every force has an equal and opposite force , it 's not clear that it has to be on different objects . but it does have to be on different objects . so these newtons force law pairs , often times is called force pairs , or newton 's third law partner forces , are always on different objects . so the convention i 'm using is that the first letter represents the object that the force is on . so this a represents that this green force f this green force f , is on a and it 's exerted by b . and this shows that it 's exerted on b , because the first letter 's on the first one , and it 's exerted by the second object , a . so this pink force is exerted on b . this green force is exerted on a . they 're equal and opposite , they do not cancel , they can not cancel because they 're not on the same object . so that 's why these do n't cancel . and they are the same magnitude , even if the two objects are not the same size . this is another misconception , if object a is a planet , a big planet . or maybe a star , this is yellow , it looks like a star . let 's say this is some big star , and this is some smaller planet orbiting that star . this is not to scale , unless this planet was enormous . so this is some planet , but this planet could be hundreds , thousands of times , millions of times less massive than this star but it would still exert the same force . so if this star is pulling on the planet with this pink force negative f , then this planet has to be pulling on the star with this green force f and they have to have the same magnitude , even if they are different sizes . so people quote newtons 's third law , but sometimes they do n't really believe it . if i told you this planet was a million times less massive than this star , people would want to say that well , then the star obviously pulls more on the planet , than the planet pulls on the star . but that 's not true according to newtons 's third law . and newtons 's third law says that they have to be the same , even if they 're different sizes . so if this was the earth and this was the moon , the earth pulls on the moon , just as much as the moon pulls on the earth . and you might still object , you might say , wait that makes no sense , i know the star just basically sits there and the planet gets whipped around in a circle . how come this planet 's getting whipped around and the star 's just staying put ? that 's because , just because the forces are equal , that does n't mean that the result is equal . in other words , the forces could be equal , but the accelerations do n't have to be equal . acceleration is gon na be the net force divided by the mass . so even if the force is the same , you divide by that mass , you 'll get a different acceleration and that 's why the result of the force does not have to be the same , even though the forces do have to be the same , because of newtons 's third law . another misconception people sometimes make , is they think there might be a delay in the creation of this newtons 's third law partner force . and people think , maybe if i exert this first force fast enough , i can catch the universe sleeping , and there might be some sort of delay in the creation of this other force . but that 's not true , newtons 's third law is universal . no matter what the situation , no matter what the acceleration or non acceleration , or motion or no motion , whether one object is bigger or smaller , if their newtons 's third law partner forces , they are equal they are opposite and they are always equal and opposite , at every given moment in time . so even if i came in all guns a blazing , chuck norris style , trying to dropkick some wall . that does not look like the correct form for a drop kick . but even if i came in , flying at this wall , as soon as i start to make contact with the wall , i 'm gon na exert a force on the wall , and the wall has to exert a force back . so i 'd exert a force on the wall to the right . and this would be the force on the wall , by my foot . there 'd have to be an equal and opposite force instantly transmitted backwards , on my foot . so this would be the force on my foot , by the wall . this happens instantaneously , there is no delay . you ca n't kick this wall fast enough , for this other force to not be generated instantaneously . as soon as your foot starts to exert any force on the wall what so ever , the wall is gon na start exerting that same force back on your foot . so newtons 's third law is universal , but people still have trouble identifying these third law partner forces . so one of the best ways to do it , is by listing both objects , as soon as you list both objects , well to figure out where the partner force is , you can just reverse these labels . so i know over here , if one of my forces is the force on the wall by my foot , to find the partner force to this force , i can just reverse the labels and say it 's got ta be the force on my foot , by the wall , which i drew over here . so this is a great way to identify the third law partner forces , cause it 's not always obvious what force is the partner force . so to show you how this can be tricky , consider this example . say we got the ground and a table . so this example drives people crazy for some reason . if i 've got a box sitting on a table , we 'll call it box a . box a is gon na have forces exerted on it . one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true . and if this box a is just sitting here , not accelerating , these two forces are going to be equal and opposite . so it 's even more tempting to say that these two forces are equal and opposite because of the third law , but that 's not true . these two forces are equal and opposite because of the second law . the second law says if there 's no acceleration , then the net force has to be zero , the forces have to cancel . and that 's what 's happening here . these forces are equal and opposite , they 're canceling on box a . which is a way to know that they are not third law partner forces , cause third law partner forces are always exerted on different objects . they can never cancel if they 're third law partner forces . so what 's going on over here ? we 've got two forces that are canceling , that are equal and opposite , but they 're not third law partner forces , they 're partner forces are somewhere else . i have n't drawn their partner forces yet . so let 's try to figure out what they 're partner forces are . so let 's get rid of this , let 's come back to here , let 's slow it down to figure out what the partner force is , name the two objects interacting . so this force of gravity , i should n't be vague , i should call it the force on object a , our box a exerted by , well you ca n't just say gravity . gravity is not an object . so the object that is exerting this gravitational force on a , is the earth . so this force really , this gravitational force , if i wan na be careful , is the force on object a exerted by the earth . now it 's easy to figure out where the partner force is . the partner force can be found just by reversing these labels . so instead of the force on a by the earth , there 's got ta be an equal and opposite force , which is the force on the earth , by box a . so opposite means it has to point up . so it has to be an upward force . and that upward force has to be exerted on the earth , by box a , and this is kind of weird , because you may not have realized it , but if the earth is pulling down on a box , or you , that means you are pulling up on the earth . and this might seem ridiculous , i mean if you jump up , you jump up , you fall back down , you move around , but the earth just sits there . if your forces are equal , how come the earth does n't move around like you do . and again , it 's because just because the forces are the same , the acceleration does n't have to be the same . the mass of the earth is so big , compared to your mass , there 's basically no acceleration . even though the forces on you and the forces on the earth are the same . so these two are third law partner forces . these two are joined together forever . they have to be equal , no matter what happens , these two forces will always be equal . i do n't care if this box is accelerating or not accelerating , or that there 's motion or no motion . whether it 's hitting a wall , sitting on a table , falling through space , these two forces must always be equal and opposite , because of the third law . so how about this other force , this force that the table was exerting . so this is , the force on a by the table . so if i wan na label it correctly , i 'd call it the force on box a , exerted by the table . now finding the third law partner force is easy , i can just reverse these labels , and i 'd get that there must be , instead of an upwards force , a downwards force on the table , by a . so i 'm gon na have another force here on the table . it 's gon na be a downward force . downward force on the table by a , that 's the third law partner force to this upward force that the table is exerting . these two forces are also third law partner forces . these forces are going to be equal and opposite no matter what happens . this force on box a by the table . and this force on the table by box a must be equal no matter what happens , but the force on box a by the table , does not have to be equal and opposite to the force on a by the earth . it happens to be equal and opposite , in a case where there 's no acceleration . if we stuck this whole situation into an elevator , or a rocket that had some huge acceleration upwards , even if there 's acceleration upwards , these partner forces have to be equal . so the force on a by the table , and the force on the table by a will have to be equal . similarly the force on the earth by a , and the force on a by the earth have to be equal . but no longer will these two forces have to be equal , cause they 're not partner forces . they might be equal and opposite in some circumstances , but they do n't always have to be equal and opposite . if we 're accelerating upwards , this upward force on the box , must be bigger than the downwards force on the box . so these wo n't be equal . recapping quickly , newtons 's third law is a statement about the forces on two different objects . and because it 's about two different objects , those forces can never cancel . to find the newtons 's third law partner force , just reverse the label after you 've identified the two objects that are interacting . the third law partner forces have to be equal in magnitude , even if one object is larger than the other , or has more charge or any property that might seem like it would convey more force , than another object . if those are the two objects interacting , their forces must be of equal magnitude and opposite directions , the forces instantaneously generated this partner forces . and be careful , some forces might seem like partner forces , and might be equal and opposite , but they 're not necessarily third law partner forces . they made just be equal and opposite for other reasons .
one of those forces is gon na be the gravitational force . so the force of gravity is gon na pull straight down on box a , and if i were to ask you , what force is the third law partner force to this force of gravity , i 'm willing to bet a lot of people might say , well there 's an upwards force on box a , exerted by the table . and that 's true .
then how is it that even the most brittle objects can withstand the gravity force at rest without a dent ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope .
can i denote that two lines are parallel as 2x+5||2x-6 or can this be interpreted as something else ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there .
okay , so the slope of a horizontal line is always going to be '0 ' , and the slope of a vertical line will always be 'undefined ' , is that right ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope .
how do i know , if the lines are not parallel , at which coordinates they intersect at ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
what is the slope of a line thats horizontal and a line thats vertical ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 .
what is the best way to redeemer standard form of a slope intercept ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
how do i find the equation of a parallel line ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope .
so what are the definitions of perpendicular lines from equation , parallel lines from equation , and perpendicular lines from the equation sal khan ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there .
what is y=2/3x +2 ; ( 9 , -3 ) ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept .
so if i was given the line y=2x+5 , how would i make an equation with the coordinate pair of ( -3 , 5 ) ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect .
what is the grade standards ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 .
what if the answers are n't in slope intercept form ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
how do you graph two parallel line from one equation ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope .
is it possible to find out where lines intersect at ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 .
how are the number can intercept ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this .
what is parallel to y=x+4 ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 .
write the equation of the line who is parallel to x=4 and contain the point ( 0,4 ) if you are given a type of problem without a given slope , how should you solve it ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 .
why is the intercept significant ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope .
why are two identical lines not considered parallel ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 .
is there another name for slope-intercept form ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 .
in the video the part around 0 if the x goes up by 1 and then y goes up by 3 , how did sal get if x goes up 2 , y goes up by 6 ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so how would i write an equation in standard form for a line that runs parallel and through a given point ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept .
so how long will this take to teach a 5 year old ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 .
what is the difference between slope intercept and y-intercept ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 .
why is the formula y=mx+b ?
we are asked which of these lines are parallel . so parallel lines are lines that have the same slope , and they 're different lines , so they never , ever intersect . so we need to look for different lines that have the exact same slope . and lucky for us , all of these lines are in y equals mx plus b or slope-intercept form , so you can really just look at these lines and figure out their slope . the slope for line a , m is equal to 2 . we see it right over there . for line b , our slope is equal to 3 , so these two guys are not parallel . i 'll graph it in a second and you 'll see that . and then finally , for line c -- i 'll do it in purple -- the slope is 2 . so m is equal to 2 . i do n't know if that purple is too dark for you . so line c and line a have the same slope , but they 're different lines , they have different y-intercepts , so they 're going to be parallel . and to see that , let 's actually graph all of these characters . so line a , our y-intercept is negative 6 . so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction . one in x , up 2 in y , if we go to in x , we 're going to go up 4 in y . and i can just do up 2 , then we 're going to go 2 , 4 , and you 're going to see it 's all on the same line , so line a is going to look something like -- do my best to draw it as straight as possible . line a -- i can do a better version than that -- line a is going to look like -- well , that 's about just as good as what i just drew -- that is line a . now let 's do line b . line b , the y-intercept is negative 6 . 0 , negative 6 . so it has the same y-intercept , but its slope is 3 , so if x goes up by 1 , y will go up by 3 . so x goes up by 1 , y goes up by 3 . if x goes up by 2 , y is going to go up by 6 . 2 , 4 , 6 . so this line is going to look something like this . trying my best to connect the dots . it has a steeper slope , and you see that when x increases , this blue line increases by more in the y direction . so that is line b -- and notice , they do intersect , there 's definitely not two parallel lines . and then finally , let 's look at line c. the y-intercept is 5 . so 0 , 1 , 2 , 3 , 4 , 5 . the point 0 , 5 , its y-intercept . and its slope is 2 . so you increase by 1 in the x direction , you 're going to go up by 2 in the y direction . if you decrease by 1 , you 're going to go down 2 in the y direction . if you increase by , well , you 're going to go to that point , you 're going to have a bunch of these points . and then if i were to graph the line -- let me do it one more time -- if i were to decrease by two , i 'm going to have to go down by 4 , right ? negative 4 over negative 2 still a slope of 2 , so 1 , 2 , 3 , 4 . and i can do that one more time , get right over there . and then you 'll see the line . the line will look like that , it will look just like that . and notice that line c and line a never intersect . they have the exact same slope . different y-intercepts , same slope , so they 're increasing at the exact same rate , but they 're never going to intersect each other . so line a and line c are parallel .
so the point 0 , 1 , 2 , 3 , 4 , 5 , 6 . and our slope is 2 . so if we move 1 in the positive x direction , we go up 2 in the positive y direction .
how do you plot a graph with a slope of -2.58 ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon .
why do we always have to set x ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit .
is n't it just easier to just divide 18 by 2.25 ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly .
is the constant of variation the same as slope ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
we do n't know what the rate is . k tells us the rate . if x goes down , y will be down .
why is the constant called k ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x .
how would you do direct variation ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
we do n't know what the rate is . k tells us the rate . if x goes down , y will be down .
why is the constant represented as `` k '' ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x .
what are all the forms of direct variation and inverse variation ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x .
can direct variation also be called direct proportions ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon .
how would you create a function rule from an x and y chart ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ?
where is information on this type of problem when there are no numbers only letters with no given values ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ?
how to get correct percentage portion to mix to get given number of liters of a percentage solution ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon .
im confused can direct variation be a fraction where x is over another number ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
40 kg of rice lasts 30 days in a family of 8 persons.if 2 guests stay with the family , how many days will 40 kg of rice last ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there .
how can you identify k if x is not 1 ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up .
suppose t varies directly with s and inversely with r square , how does the value of t change when the value of s is doubled ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ?
can someone give me another example of a constant ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs .
when you have an equation like 2y=5x+1 , is there a direct variation ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 .
can you explain why y=2x+1 would n't be a direct variation ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x .
what is a direct variation ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 .
what if i had a problem like 8x + 9y = 10 , what would you do when dividing the 8x that you have already subracted from both sides like 9y = 10 -8x ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 .
im still confused , lets say 4 cups of flour are needed to make 24 rolls , how much flour is needed for 36 rolls ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction .
how would i write an equation if for example the # of cats varies directly to the # of dogs , if there is 10 dogs and 15 cats how would i write that equation ?
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x . so that means that y is equal to some constant , we 'll just call that k , times x . this is what it means to vary directly . if x goes up , y will go up . we do n't know what the rate is . k tells us the rate . if x goes down , y will be down . now , they give us more information , and this will help us figure out what k is . if a gallon of gas costs $ 2.25 , how many gallons could you purchase for $ 18 ? so if x is equal to 1 -- this statement up here , a gallon of gas -- that tells us if we get 1 gallon , if x is equal to 1 , then y is $ 2.25 , right ? y is what it costs . they tell us 1 gallon costs $ 2.25 , so you could write it right here , $ 2.25 is equal to k times x , times 1 . well , i did n't even have to write the times 1 there . it 's essentially telling us exactly what the rate is , what k is . we do n't even have to write that 1 there . k is equal to 2.25 . that 's what this told us right there . so the equation , how y varies with x , is y is equal to 2.25x , where x is the number of gallons we purchase . y is the cost of that purchase , so it 's $ 2.25 a gallon . and then they ask us , how many gallons could you purchase for $ 18 ? so $ 18 is going to be our total cost . it is y cost of filling the car . so 18 is going to be equal to 2.25x . now if we want to solve for x , we can divide both sides by 2.25 , so let 's do that . you divide 18 by 2.25 , divide 2.25x by 2.25 , and what do we get ? let me scroll down a little bit . the right-hand side , the 2.25 's cancel out , you get x . and then what is 18 divided by 2.25 ? so let me write this down . so first of all , i just like to think of it as a fraction . 2.25 is the same thing -- let me write over here -- 2.25 is equal to 2 and 1/4 , which is the same thing as 9 over 4 . so 18 divided by 2.25 is equal to 18 divided by 9 over 4 , which is equal to 18 times 4 over 9 , or 18 over 1 times 4 over 9 . and let 's see , 18 divided by 9 is 2 , 9 divided by 9 is 1 . that simplifies pretty nicely into 8 . so 18 divided by 2.25 is 8 , so we can buy 8 gallons for $ 18 .
we 're told that the total cost of filling up your car with gas varies directly with the number of gallons of gasoline you are purchasing . so this first statement tells us that if x is equal to the number of gallons purchased , and y is equal to the cost of filling up the car , this first statement tells us that y varies directly with the number of gallons , with x .
what is after direct variation ?