[{"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "In this video, we're going to talk about analog versus digital. Something that's analog can be any value within a given range, while something digital is represented by a number of discrete or separate levels. To distinguish these two ideas, I like to think about clocks. An analog clock has the numbers in the hands, and it's analog because the motion of those hands is continuous. They can sweep across the circle, representing any of infinite times on that clock. For example, between 3.06 and 3.07, the minute hand is actually going to be at some point between those marks on the clock, showing one of the infinitely possible times that the clock can represent. Compare that to a digital clock."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "An analog clock has the numbers in the hands, and it's analog because the motion of those hands is continuous. They can sweep across the circle, representing any of infinite times on that clock. For example, between 3.06 and 3.07, the minute hand is actually going to be at some point between those marks on the clock, showing one of the infinitely possible times that the clock can represent. Compare that to a digital clock. A digital clock is only going to show you 3.06 or 3.07. It will never display any of the many fractional seconds between those two times. Digital only takes on certain discrete values, and it has a finite number of those values."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "Compare that to a digital clock. A digital clock is only going to show you 3.06 or 3.07. It will never display any of the many fractional seconds between those two times. Digital only takes on certain discrete values, and it has a finite number of those values. So an analog wave or signal will smoothly sweep across the infinitely many possible values it has, while a digital wave or signal will only be at one of a number of discrete values, so the shape of the wave will be more square or step-like. Let's check out an example so this makes a little more sense. I like music, so we're going to talk about sound."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "Digital only takes on certain discrete values, and it has a finite number of those values. So an analog wave or signal will smoothly sweep across the infinitely many possible values it has, while a digital wave or signal will only be at one of a number of discrete values, so the shape of the wave will be more square or step-like. Let's check out an example so this makes a little more sense. I like music, so we're going to talk about sound. Sound is an analog signal or wave. So if we look at a graph of sound, volume over time, it's going to have a smooth, continuous analog waveform. Both the amplitude, or the volume, and the frequency, what we hear as pitch, are changing continuously between infinite possible values."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "I like music, so we're going to talk about sound. Sound is an analog signal or wave. So if we look at a graph of sound, volume over time, it's going to have a smooth, continuous analog waveform. Both the amplitude, or the volume, and the frequency, what we hear as pitch, are changing continuously between infinite possible values. And that's because sound waves, the vibration of particles propagating through the air, actually changes continuously. The very first sound recording and reproduction technology imprinted that analog wave directly onto a material. For example, records imprint that sound wave into vinyl, and cassettes imprint the sound wave onto tape."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "Both the amplitude, or the volume, and the frequency, what we hear as pitch, are changing continuously between infinite possible values. And that's because sound waves, the vibration of particles propagating through the air, actually changes continuously. The very first sound recording and reproduction technology imprinted that analog wave directly onto a material. For example, records imprint that sound wave into vinyl, and cassettes imprint the sound wave onto tape. A major drawback of this technology is that for the sound to play back exactly as it was recorded, that waveform needs to stay untouched, right? So think about scratching vinyl, or stretching or smudging a cassette tape. That's directly deforming the wave, so you'll never be able to reproduce the sound exactly as it was recorded."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "For example, records imprint that sound wave into vinyl, and cassettes imprint the sound wave onto tape. A major drawback of this technology is that for the sound to play back exactly as it was recorded, that waveform needs to stay untouched, right? So think about scratching vinyl, or stretching or smudging a cassette tape. That's directly deforming the wave, so you'll never be able to reproduce the sound exactly as it was recorded. So technology advanced, and sound waves became digitized. Here's how. Alright, so recall our analog sound wave."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "That's directly deforming the wave, so you'll never be able to reproduce the sound exactly as it was recorded. So technology advanced, and sound waves became digitized. Here's how. Alright, so recall our analog sound wave. We have a smooth analog wave that's taking on any number of infinitely possible values within this range. In order to digitize this wave, we're going to ascribe numbers to the amplitude at different points. Alright?"}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "Alright, so recall our analog sound wave. We have a smooth analog wave that's taking on any number of infinitely possible values within this range. In order to digitize this wave, we're going to ascribe numbers to the amplitude at different points. Alright? Watch this magic. So we go over here and make a scale. So we're breaking up the amplitudes into discrete possibilities."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "Alright? Watch this magic. So we go over here and make a scale. So we're breaking up the amplitudes into discrete possibilities. Then we can go through the wave, and at specific points of the wave, measure what is the amplitude based on that scale. So over here, we're at the first point of the scale. At this peak, we're at the second point of our scale."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "So we're breaking up the amplitudes into discrete possibilities. Then we can go through the wave, and at specific points of the wave, measure what is the amplitude based on that scale. So over here, we're at the first point of the scale. At this peak, we're at the second point of our scale. Then the first, the third, the second, the fourth, back down to the first. Now that we have this wave broken up into discrete levels, right, we can ascribe the numbers and we effectively turn this analog wave into a set of numbers. One, two, one, three, two, four, one."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "At this peak, we're at the second point of our scale. Then the first, the third, the second, the fourth, back down to the first. Now that we have this wave broken up into discrete levels, right, we can ascribe the numbers and we effectively turn this analog wave into a set of numbers. One, two, one, three, two, four, one. Our wave has been digitized. Now that digitized wave can be played back through a speaker to recreate the analog wave. As long as the sampling happens at a quick enough rate, humans can't tell the difference."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "One, two, one, three, two, four, one. Our wave has been digitized. Now that digitized wave can be played back through a speaker to recreate the analog wave. As long as the sampling happens at a quick enough rate, humans can't tell the difference. Alright? So the digitization of waves is all about ascribing specific numbers to some of those mechanical properties of the wave. The important thing here is that now that the wave has been digitized, the digitized sound wave can be reliably stored, processed, and communicated with computers."}, {"video_title": "Digital and analog information   Information Technologies   High School Physics   Khan Academy.mp3", "Sentence": "As long as the sampling happens at a quick enough rate, humans can't tell the difference. Alright? So the digitization of waves is all about ascribing specific numbers to some of those mechanical properties of the wave. The important thing here is that now that the wave has been digitized, the digitized sound wave can be reliably stored, processed, and communicated with computers. So some information is lost in translation, but once the wave is digitized, its quality will never degrade. Okay? This allows for a lot more reliable technology because the wave is represented with numbers instead of it being physically imprinted on some material."}, {"video_title": "Potential energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Hello everyone, let's talk about potential energy. Potential energy is energy that is stored in an object and this energy is related to the potential or the future possibility for an object to have a different type of energy like kinetic energy for motion that is converted from that potential energy. There are many kinds of potential energy, but they all arise from an object's relation to a position or an original shape. So while in general there are many different types of potential energy, there are several specific types that are very common, so let's talk about these. Gravitational potential energy is the potential energy that an object with mass has due to the force of gravity from another object with mass, like say the Earth. And in fact we often use the surface of the Earth to compare an object's position with to see how much potential energy it has in the Earth's gravitational field. Gravity is an attractive force, so objects with mass want to move towards the surface of the Earth."}, {"video_title": "Potential energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "So while in general there are many different types of potential energy, there are several specific types that are very common, so let's talk about these. Gravitational potential energy is the potential energy that an object with mass has due to the force of gravity from another object with mass, like say the Earth. And in fact we often use the surface of the Earth to compare an object's position with to see how much potential energy it has in the Earth's gravitational field. Gravity is an attractive force, so objects with mass want to move towards the surface of the Earth. If we move them further away or opposite the direction of the gravitational force, we increase their gravitational potential energy, and the opposite is true if it gets closer. When an object is on the surface of Earth, we typically say it has no potential energy, but you could use any point to be this comparison where potential energy is zero. Consider a book on a bookshelf."}, {"video_title": "Potential energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Gravity is an attractive force, so objects with mass want to move towards the surface of the Earth. If we move them further away or opposite the direction of the gravitational force, we increase their gravitational potential energy, and the opposite is true if it gets closer. When an object is on the surface of Earth, we typically say it has no potential energy, but you could use any point to be this comparison where potential energy is zero. Consider a book on a bookshelf. If the book is on this shelf, we can use this shelf as the 0.4 potential energy. Moving it to a higher shelf would mean it has gravitational potential energy relative to that lower shelf, or relative to the floor if we want to use that as our comparison instead. Next we have elastic potential energy, which is the potential energy some objects have due to their shape being changed."}, {"video_title": "Potential energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Consider a book on a bookshelf. If the book is on this shelf, we can use this shelf as the 0.4 potential energy. Moving it to a higher shelf would mean it has gravitational potential energy relative to that lower shelf, or relative to the floor if we want to use that as our comparison instead. Next we have elastic potential energy, which is the potential energy some objects have due to their shape being changed. These types of objects are called elastic objects. Elastic objects are made of materials and designed so they have internal or inside forces that try to return them to their original shape. One very common example of this is a spring."}, {"video_title": "Potential energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Next we have elastic potential energy, which is the potential energy some objects have due to their shape being changed. These types of objects are called elastic objects. Elastic objects are made of materials and designed so they have internal or inside forces that try to return them to their original shape. One very common example of this is a spring. When you stretch or compress a spring, you change its shape, and the shape of the spring causes internal forces that try to return the spring to its original shape. Now electric potential energy, which is the potential energy a charged object has due to the electric force from another charged object. Opposite electric charges are attracted to one another, and similar electric charges are repelled, so the potential energy depends on what type of charges there are and how far apart they are."}, {"video_title": "Potential energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "One very common example of this is a spring. When you stretch or compress a spring, you change its shape, and the shape of the spring causes internal forces that try to return the spring to its original shape. Now electric potential energy, which is the potential energy a charged object has due to the electric force from another charged object. Opposite electric charges are attracted to one another, and similar electric charges are repelled, so the potential energy depends on what type of charges there are and how far apart they are. Potential energy increases when the charges move opposite the direction of the electric force, for example when two negative charges get closer together. Similarly, magnetic potential energy is the potential energy a magnetic object has due to the magnetic force from another magnet. Magnetic force causes similar poles to repel one another and opposite poles to attract, and because magnets have north and south poles, the potential energy depends not only on the position within a field, but also the magnet's orientation."}, {"video_title": "Potential energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Opposite electric charges are attracted to one another, and similar electric charges are repelled, so the potential energy depends on what type of charges there are and how far apart they are. Potential energy increases when the charges move opposite the direction of the electric force, for example when two negative charges get closer together. Similarly, magnetic potential energy is the potential energy a magnetic object has due to the magnetic force from another magnet. Magnetic force causes similar poles to repel one another and opposite poles to attract, and because magnets have north and south poles, the potential energy depends not only on the position within a field, but also the magnet's orientation. Again, you could increase the potential energy by moving the magnets opposite the direction of the magnetic force, for example by pulling apart a north pole and a south pole. All of these types of energy are due to different forces and are calculated differently from different equations, which we won't cover here, but they are all potential energy. And these are just a few of the most common types of potential energy, but there are more."}, {"video_title": "Potential energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Magnetic force causes similar poles to repel one another and opposite poles to attract, and because magnets have north and south poles, the potential energy depends not only on the position within a field, but also the magnet's orientation. Again, you could increase the potential energy by moving the magnets opposite the direction of the magnetic force, for example by pulling apart a north pole and a south pole. All of these types of energy are due to different forces and are calculated differently from different equations, which we won't cover here, but they are all potential energy. And these are just a few of the most common types of potential energy, but there are more. In summary, potential energy is the stored energy in an object due to its position, its properties, and the forces acting on it. Potential energy is measured relative to some comparison position or shape and describes the potential for other forms of energy, commonly kinetic energy for motion, to exist. There are many forms of potential energy, including gravitational, elastic, magnetic, and electric."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And this is also known as Newton's third law of motion, but it's also one of the most misunderstood laws of physics. So that's why we're going to dig into it a little bit in this video. So I have two examples here where Newton's third law or this notion of an action and a reaction force is happening. So over here, you have this plane flying and the plane is able to move forward by pushing air particles through these jet engines. So these air particles are pushed outward at a very, very high velocity out the back of the engines. If you were to enlarge one of those air particles, let's say this is this purple dot right over here, there is a force that is being exerted on it by the jet engine. And that force is going in that direction."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "So over here, you have this plane flying and the plane is able to move forward by pushing air particles through these jet engines. So these air particles are pushed outward at a very, very high velocity out the back of the engines. If you were to enlarge one of those air particles, let's say this is this purple dot right over here, there is a force that is being exerted on it by the jet engine. And that force is going in that direction. So what is the equal and opposite reaction force? Well, the equal and opposite reaction force is not also occurring on that molecule, it's what the molecule is doing to the plane. The equal and opposite reaction force is that the molecule is going to be pushing on the jet engine with an equal, but an opposite force."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And that force is going in that direction. So what is the equal and opposite reaction force? Well, the equal and opposite reaction force is not also occurring on that molecule, it's what the molecule is doing to the plane. The equal and opposite reaction force is that the molecule is going to be pushing on the jet engine with an equal, but an opposite force. So it's going to go in the opposite direction. And that's how the jet is able to accelerate forward by pushing on these particles and accelerating them backward by exerting a force on them. The equal and opposite force is the force that the particles, those molecules of air are exerting on the jet and moving it forward."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "The equal and opposite reaction force is that the molecule is going to be pushing on the jet engine with an equal, but an opposite force. So it's going to go in the opposite direction. And that's how the jet is able to accelerate forward by pushing on these particles and accelerating them backward by exerting a force on them. The equal and opposite force is the force that the particles, those molecules of air are exerting on the jet and moving it forward. The same thing here is going on with this rocket. You have some rocket fuel in there, it gets ignited, it explodes. And as it explodes, there's a force that exerts on those little molecules."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "The equal and opposite force is the force that the particles, those molecules of air are exerting on the jet and moving it forward. The same thing here is going on with this rocket. You have some rocket fuel in there, it gets ignited, it explodes. And as it explodes, there's a force that exerts on those little molecules. And that force is going in this direction. But as it does that, there's an equal and opposite force that the molecules are exerting on the rocket. The rocket is having a force acted on it, once again, equal and opposite."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And as it explodes, there's a force that exerts on those little molecules. And that force is going in this direction. But as it does that, there's an equal and opposite force that the molecules are exerting on the rocket. The rocket is having a force acted on it, once again, equal and opposite. So it's important to realize that the reaction force is not on the same object, it's on the other object. If one object is putting an action force on another, then the second object is putting a reaction force on the first. The forces do not cancel out."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "The rocket is having a force acted on it, once again, equal and opposite. So it's important to realize that the reaction force is not on the same object, it's on the other object. If one object is putting an action force on another, then the second object is putting a reaction force on the first. The forces do not cancel out. It's also important to realize that both forces are generated in pairs and happen at the exact same time. There's no delay. We can look at other examples of this."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "The forces do not cancel out. It's also important to realize that both forces are generated in pairs and happen at the exact same time. There's no delay. We can look at other examples of this. This is a scenario that I would never want to be caught in being drifting through space. Now, this astronaut here has some type of a rocket pack that might help it move around. But let's say your rocket pack ran out of fuel and you're just drifting through space."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "We can look at other examples of this. This is a scenario that I would never want to be caught in being drifting through space. Now, this astronaut here has some type of a rocket pack that might help it move around. But let's say your rocket pack ran out of fuel and you're just drifting through space. How can you get back to your spaceship? Well, if you have a wrench or something on you that you can throw, if you can take that wrench and if you can push that wrench in that direction, and let's say your spaceship is over here to the left, well, the equal and opposite force is the force that the wrench is going to exert on you, the astronaut, and then it will push you in that direction and accelerate you in that direction. So that's a useful thing if you ever get caught drifting through space."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "But let's say your rocket pack ran out of fuel and you're just drifting through space. How can you get back to your spaceship? Well, if you have a wrench or something on you that you can throw, if you can take that wrench and if you can push that wrench in that direction, and let's say your spaceship is over here to the left, well, the equal and opposite force is the force that the wrench is going to exert on you, the astronaut, and then it will push you in that direction and accelerate you in that direction. So that's a useful thing if you ever get caught drifting through space. But you could do an experiment right now. Press on the table in front of you. When you press on that table, you're clearly putting a force onto that table."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "So that's a useful thing if you ever get caught drifting through space. But you could do an experiment right now. Press on the table in front of you. When you press on that table, you're clearly putting a force onto that table. If your table is soft, you'll see it get compressed. But notice your finger itself is also getting compressed. And the whole reason why you can even feel that is because your finger is getting compressed."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "When you press on that table, you're clearly putting a force onto that table. If your table is soft, you'll see it get compressed. But notice your finger itself is also getting compressed. And the whole reason why you can even feel that is because your finger is getting compressed. And that is the equal and opposite force that the table is putting on your finger. And this can happen at very, very large distances as well. The whole reason why the moon is in orbit around the Earth is because there's a gravitational force of Earth's mass acting on the moon, but there's an equal and opposite force of the moon acting on Earth."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And the whole reason why you can even feel that is because your finger is getting compressed. And that is the equal and opposite force that the table is putting on your finger. And this can happen at very, very large distances as well. The whole reason why the moon is in orbit around the Earth is because there's a gravitational force of Earth's mass acting on the moon, but there's an equal and opposite force of the moon acting on Earth. And it's actually not that the moon is rotating around the Earth. It's actually they're both rotating around the center of mass of their combination. That just happens to be so much closer to Earth."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "The whole reason why the moon is in orbit around the Earth is because there's a gravitational force of Earth's mass acting on the moon, but there's an equal and opposite force of the moon acting on Earth. And it's actually not that the moon is rotating around the Earth. It's actually they're both rotating around the center of mass of their combination. That just happens to be so much closer to Earth. It's actually within Earth's volume that it looks like the moon is rotating around the Earth. And this isn't just celestial bodies. I weigh 165 pounds."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "That just happens to be so much closer to Earth. It's actually within Earth's volume that it looks like the moon is rotating around the Earth. And this isn't just celestial bodies. I weigh 165 pounds. That is the force that Earth is acting on me due to gravity. But it turns out that there's an equal and opposite force of 165 pounds that I am pulling on Earth with. So I will leave you there."}, {"video_title": "Action and reaction forces   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "I weigh 165 pounds. That is the force that Earth is acting on me due to gravity. But it turns out that there's an equal and opposite force of 165 pounds that I am pulling on Earth with. So I will leave you there. Look around the world. This is happening everywhere. For every force, there's an equal and opposite reaction force."}, {"video_title": "Frames of reference   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "We do this by using units and frames of reference, which are also called reference frames. We talk about units in another video, so let's look at what a frame of reference is. Let's say this blue box thing is a car, and it's going 45 miles per hour. Someone standing on the side of the road would see it pass at 45 miles per hour. Now if this yellow truck is going 40 miles per hour, someone sitting in the yellow truck would observe the blue car traveling at five miles per hour. How could the person on the side of the road see the blue car traveling at 45 miles per hour, and a person in the yellow truck see the blue car moving at five miles per hour? This is because both observers are using different frames of reference."}, {"video_title": "Frames of reference   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Someone standing on the side of the road would see it pass at 45 miles per hour. Now if this yellow truck is going 40 miles per hour, someone sitting in the yellow truck would observe the blue car traveling at five miles per hour. How could the person on the side of the road see the blue car traveling at 45 miles per hour, and a person in the yellow truck see the blue car moving at five miles per hour? This is because both observers are using different frames of reference. So let's go ahead and take a look at that, starting with the speed of the blue car. The person on the side of the road is using their frame of reference of being at rest, so relative to them, the blue car is moving at 45 miles per hour. To the person in this yellow truck, which remember is already going 40 miles per hour, the blue car is going five miles per hour."}, {"video_title": "Frames of reference   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "This is because both observers are using different frames of reference. So let's go ahead and take a look at that, starting with the speed of the blue car. The person on the side of the road is using their frame of reference of being at rest, so relative to them, the blue car is moving at 45 miles per hour. To the person in this yellow truck, which remember is already going 40 miles per hour, the blue car is going five miles per hour. Now let's do the exact same thing for the speed of the yellow truck. So what is the speed of the yellow truck for the observer on the side of the road? It's 40 miles per hour."}, {"video_title": "Frames of reference   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "To the person in this yellow truck, which remember is already going 40 miles per hour, the blue car is going five miles per hour. Now let's do the exact same thing for the speed of the yellow truck. So what is the speed of the yellow truck for the observer on the side of the road? It's 40 miles per hour. And what do you think the speed of the truck is for the person using their blue car as the reference frame? Well, the blue car is moving at 45 miles per hour, and the truck is only moving at 40 miles per hour. So the speed of the yellow truck is actually five miles per hour slower than this reference frame because the blue car is already moving at 45 miles per hour."}, {"video_title": "Frames of reference   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "It's 40 miles per hour. And what do you think the speed of the truck is for the person using their blue car as the reference frame? Well, the blue car is moving at 45 miles per hour, and the truck is only moving at 40 miles per hour. So the speed of the yellow truck is actually five miles per hour slower than this reference frame because the blue car is already moving at 45 miles per hour. Now you might be thinking, but wait, the person on the side of the road isn't really at rest. They're on the earth and the earth is moving. You're completely correct."}, {"video_title": "Frames of reference   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "So the speed of the yellow truck is actually five miles per hour slower than this reference frame because the blue car is already moving at 45 miles per hour. Now you might be thinking, but wait, the person on the side of the road isn't really at rest. They're on the earth and the earth is moving. You're completely correct. The person is at rest with respect to the earth, and the earth is the most common frame of reference that we use. To an observer in space who is not rotating with the earth, the blue car is going 45 miles per hour plus the speed of earth's rotation. And this is why frame of reference is so important."}, {"video_title": "Frames of reference   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "You're completely correct. The person is at rest with respect to the earth, and the earth is the most common frame of reference that we use. To an observer in space who is not rotating with the earth, the blue car is going 45 miles per hour plus the speed of earth's rotation. And this is why frame of reference is so important. We just talked about one blue car having three different velocities depending what the frame of reference is. How would we communicate this to avoid confusion? Well, we state the reference frame we're using."}, {"video_title": "Frames of reference   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And this is why frame of reference is so important. We just talked about one blue car having three different velocities depending what the frame of reference is. How would we communicate this to avoid confusion? Well, we state the reference frame we're using. The blue car is moving at five miles per hour with respect to, which I'll write as WRT, the yellow truck. This tells us that the yellow truck is our frame of reference. Or we could say that the yellow truck is moving at 40 miles per hour and the blue car at 45 miles per hour with respect to the earth."}, {"video_title": "Net force   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And what we wanna do is we wanna start to move it. So what we do is we attach a rocket to one side and then we ignite that rocket and it starts to send all the superheated gas, all of these particles to the right. Well, what do you think that's going to do to the asteroid? Well, it's going to push on the asteroid in that direction or you could say it's going to exert a force on that asteroid. And we could show that force like this where the strength of that force or the magnitude of the force is the length of this line. And then the direction I will specify or show with that arrow. So fair enough, I will be pushing towards the left."}, {"video_title": "Net force   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Well, it's going to push on the asteroid in that direction or you could say it's going to exert a force on that asteroid. And we could show that force like this where the strength of that force or the magnitude of the force is the length of this line. And then the direction I will specify or show with that arrow. So fair enough, I will be pushing towards the left. And when I push to the left, it doesn't just start to move the asteroid to the left, it actually will accelerate the asteroid to the left. So the longer that this rocket is running, it's going to make the asteroid move to the left faster and faster and faster. But let's think about another example."}, {"video_title": "Net force   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "So fair enough, I will be pushing towards the left. And when I push to the left, it doesn't just start to move the asteroid to the left, it actually will accelerate the asteroid to the left. So the longer that this rocket is running, it's going to make the asteroid move to the left faster and faster and faster. But let's think about another example. Let's say that you and one of your friends, you had a little bit of miscommunication and they went and put an identical rocket on this side of the asteroid and y'all ignited it at the exact same time. So this one is going to push in the other direction. What do you think is going to happen if these happened at the exact same time?"}, {"video_title": "Net force   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "But let's think about another example. Let's say that you and one of your friends, you had a little bit of miscommunication and they went and put an identical rocket on this side of the asteroid and y'all ignited it at the exact same time. So this one is going to push in the other direction. What do you think is going to happen if these happened at the exact same time? Even though there's now twice as much force being exerted on this asteroid, it's going in opposite directions. So they zero out and so there's zero net force. And so this asteroid won't be accelerated at all."}, {"video_title": "Net force   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "What do you think is going to happen if these happened at the exact same time? Even though there's now twice as much force being exerted on this asteroid, it's going in opposite directions. So they zero out and so there's zero net force. And so this asteroid won't be accelerated at all. Now let's say that a third friend wanted to correct this situation and this isn't necessarily the most efficient way to do it, but what they do is they put another identical rocket right over here and let's say ignite that. Now what will happen? Well, now you had the original two forces that net out to each other, but now you have this also this new force, which I will make in purple because it's a purple rocket."}, {"video_title": "Net force   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And so this asteroid won't be accelerated at all. Now let's say that a third friend wanted to correct this situation and this isn't necessarily the most efficient way to do it, but what they do is they put another identical rocket right over here and let's say ignite that. Now what will happen? Well, now you had the original two forces that net out to each other, but now you have this also this new force, which I will make in purple because it's a purple rocket. And so that new force, you could draw like this to show, all right, that will now be the net force because you have the equivalent of two rockets going in the left direction and one rocket going in the right direction. Or another way we could draw that is we have two rockets going in the left direction. So that would have a force that looks like this."}, {"video_title": "Net force   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Well, now you had the original two forces that net out to each other, but now you have this also this new force, which I will make in purple because it's a purple rocket. And so that new force, you could draw like this to show, all right, that will now be the net force because you have the equivalent of two rockets going in the left direction and one rocket going in the right direction. Or another way we could draw that is we have two rockets going in the left direction. So that would have a force that looks like this. And then we have one going in the right direction. And so if you were to net it out, this is equivalent to just having one rocket that we originally saw. That's equivalent to just going back to what we originally saw."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Let's talk about waves. So let's imagine that you were to take a string and attach it at one end to a wall, and then on the other end, you were to wiggle it up and down. Well, then you would have made a wave. You would see a pattern that looks like this. Now, what could be a good definition for a wave? Well, we could call it a traveling disturbance. Well, what does that mean?"}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "You would see a pattern that looks like this. Now, what could be a good definition for a wave? Well, we could call it a traveling disturbance. Well, what does that mean? Well, we're disturbing the rope. If we didn't move it, if we just held it straight, it might look something like that, or it might just hang down a little bit, but clearly, we are now moving it up and down, and those movements are disturbing that rope, and that disturbance can move along that rope. Now, we see waves not just in ropes that are moving up and down."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Well, what does that mean? Well, we're disturbing the rope. If we didn't move it, if we just held it straight, it might look something like that, or it might just hang down a little bit, but clearly, we are now moving it up and down, and those movements are disturbing that rope, and that disturbance can move along that rope. Now, we see waves not just in ropes that are moving up and down. You have probably seen water waves. If you were to take a tank of water like this, and if you were to start pressing on one end of the water here, you would see these waveforms that start. We can also see that with sound and sound waves."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Now, we see waves not just in ropes that are moving up and down. You have probably seen water waves. If you were to take a tank of water like this, and if you were to start pressing on one end of the water here, you would see these waveforms that start. We can also see that with sound and sound waves. You might not realize it, but the sound of my voice right now is actually just a traveling compression or disturbance in the air that is getting to your ear, and then little hairs in your ears can sense those changes in pressure from the air, and your mind perceives that as sound, and once again, this is a traveling disturbance. You have particles that have high pressure, and then they knock into the particles next to them that then knock into the particles next to them, so if you were to be able to observe this in slow motion, you would see these high-pressure parts right over here could be traveling, say, to the right, and even though this might be a pressure wave that's traveling through the air, we can represent it in a way that looks a lot like our first rope that we were moving up and down. Areas where things are high, in the sound example, that's high pressure, and you have areas where things are low, in the sound example, that is low pressure."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "We can also see that with sound and sound waves. You might not realize it, but the sound of my voice right now is actually just a traveling compression or disturbance in the air that is getting to your ear, and then little hairs in your ears can sense those changes in pressure from the air, and your mind perceives that as sound, and once again, this is a traveling disturbance. You have particles that have high pressure, and then they knock into the particles next to them that then knock into the particles next to them, so if you were to be able to observe this in slow motion, you would see these high-pressure parts right over here could be traveling, say, to the right, and even though this might be a pressure wave that's traveling through the air, we can represent it in a way that looks a lot like our first rope that we were moving up and down. Areas where things are high, in the sound example, that's high pressure, and you have areas where things are low, in the sound example, that is low pressure. Now, when we talk about waves, there are common properties. For example, we might wanna know how much are we getting disturbed from what we would call the equilibrium. You could view that as maybe the middle state right over there."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Areas where things are high, in the sound example, that's high pressure, and you have areas where things are low, in the sound example, that is low pressure. Now, when we talk about waves, there are common properties. For example, we might wanna know how much are we getting disturbed from what we would call the equilibrium. You could view that as maybe the middle state right over there. Well, if we're getting disturbed that much, we could call that the amplitude. That's how much we are going above or below that equilibrium. This would be the amplitude as well."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "You could view that as maybe the middle state right over there. Well, if we're getting disturbed that much, we could call that the amplitude. That's how much we are going above or below that equilibrium. This would be the amplitude as well. We could think about how far is it from the same points on the wave, so if we go from one peak to another peak, well, we could call that the wavelength, and you could just do it from any one point on the wave that's just like it on the wave again, so that would be the same wavelength as our original wavelength right over there. You might hear the term frequency of a wave, and one way to think about that is if you were to just observe our original rope, and if you were to say, how many times does it go all the way up, all the way down, and then back up, so it completes a full cycle, how many times can it do that in a second? If it does that five times in a second, then someone might say it has a frequency of five cycles per second."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "This would be the amplitude as well. We could think about how far is it from the same points on the wave, so if we go from one peak to another peak, well, we could call that the wavelength, and you could just do it from any one point on the wave that's just like it on the wave again, so that would be the same wavelength as our original wavelength right over there. You might hear the term frequency of a wave, and one way to think about that is if you were to just observe our original rope, and if you were to say, how many times does it go all the way up, all the way down, and then back up, so it completes a full cycle, how many times can it do that in a second? If it does that five times in a second, then someone might say it has a frequency of five cycles per second. Now, everything that we have just talked about, these are called mechanical waves. It's a special category, probably the ones that you will see most often. Now, mechanical waves need a medium to travel through."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "If it does that five times in a second, then someone might say it has a frequency of five cycles per second. Now, everything that we have just talked about, these are called mechanical waves. It's a special category, probably the ones that you will see most often. Now, mechanical waves need a medium to travel through. In the rope example, the medium was the rope. In the water example, it's the water. In the sound example, the medium is the air."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Now, mechanical waves need a medium to travel through. In the rope example, the medium was the rope. In the water example, it's the water. In the sound example, the medium is the air. Now, there are things that can be described as waves that don't need a medium. In particular, and this is kind of mind-boggling, is that light can be considered a wave. If we think about the different frequencies of light, our brain perceives that as different colors, and if we think about the amplitude of light, our brain perceives that as the intensity of light, how bright it is."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "In the sound example, the medium is the air. Now, there are things that can be described as waves that don't need a medium. In particular, and this is kind of mind-boggling, is that light can be considered a wave. If we think about the different frequencies of light, our brain perceives that as different colors, and if we think about the amplitude of light, our brain perceives that as the intensity of light, how bright it is. And even more mind-blowing, visible light are just certain frequencies of what we would call electromagnetic waves. There's actually higher frequencies of electromagnetic waves that have all sorts of applications. You might've heard of ultraviolet light, or X-rays, or gamma rays."}, {"video_title": "Mechanical waves and light   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "If we think about the different frequencies of light, our brain perceives that as different colors, and if we think about the amplitude of light, our brain perceives that as the intensity of light, how bright it is. And even more mind-blowing, visible light are just certain frequencies of what we would call electromagnetic waves. There's actually higher frequencies of electromagnetic waves that have all sorts of applications. You might've heard of ultraviolet light, or X-rays, or gamma rays. Similarly, there are lower wavelengths of light. You might've heard things like infrared or radio waves. These are all just different frequencies of what's known as electromagnetic waves."}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And when we're talking about data and measurements with other scientists, we need to make sure we're on the same page. So how do we do that? Well, one of the ways is to use units. We use units whenever we talk about things like position, where an object's located, how long it is. Its mass, how much matter it's made up of. Or its motion, how is that object moving? You probably hear units every day."}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "We use units whenever we talk about things like position, where an object's located, how long it is. Its mass, how much matter it's made up of. Or its motion, how is that object moving? You probably hear units every day. For example, you've grown, let's say, an inch and a half in the past year. Or that tree over there is 25 feet tall. And maybe you went swimming in a 25 meter pool."}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "You probably hear units every day. For example, you've grown, let's say, an inch and a half in the past year. Or that tree over there is 25 feet tall. And maybe you went swimming in a 25 meter pool. And we're just going to pretend that the pool is a rectangle, because as you can tell from my tree, my artistic skills are not that great. Anyway, this brings up a super important point about why we use units. I just used three examples of length measurements with three different units."}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And maybe you went swimming in a 25 meter pool. And we're just going to pretend that the pool is a rectangle, because as you can tell from my tree, my artistic skills are not that great. Anyway, this brings up a super important point about why we use units. I just used three examples of length measurements with three different units. Inches, feet, and meters. Imagine if I didn't attach a unit to any of these measurements. You grew one and a half what?"}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "I just used three examples of length measurements with three different units. Inches, feet, and meters. Imagine if I didn't attach a unit to any of these measurements. You grew one and a half what? Meters? Whoa. One and a half hands?"}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "You grew one and a half what? Meters? Whoa. One and a half hands? Well, whose hands? Your hands? Or my hands?"}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "One and a half hands? Well, whose hands? Your hands? Or my hands? Oof. Well, pretend those are hands. Units let us know how much of a quantity there is."}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Or my hands? Oof. Well, pretend those are hands. Units let us know how much of a quantity there is. So a meter is always used to measure length, and we know exactly how long a meter is. That way, when we say something is two meters long, no one has to guess at how big that is. Any measurement or data point always needs to have a unit, or else it's just a meaningless number."}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Units let us know how much of a quantity there is. So a meter is always used to measure length, and we know exactly how long a meter is. That way, when we say something is two meters long, no one has to guess at how big that is. Any measurement or data point always needs to have a unit, or else it's just a meaningless number. To avoid any confusion, in science we use what are called SI units. SI units are the International System. Could there be any more letters in this word?"}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Any measurement or data point always needs to have a unit, or else it's just a meaningless number. To avoid any confusion, in science we use what are called SI units. SI units are the International System. Could there be any more letters in this word? System. Used by scientists all over the world. People use meters to describe position or length, kilograms for mass, and if we're talking about the motion of something, meters per second."}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Could there be any more letters in this word? System. Used by scientists all over the world. People use meters to describe position or length, kilograms for mass, and if we're talking about the motion of something, meters per second. And while this is the agreed upon scientific unit system, you should be aware that other systems do exist, which means things can very easily get very confusing if you forget your units. And you might be thinking, oh come on, who mixes up units? Well, it happens more often than you think."}, {"video_title": "Units   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "People use meters to describe position or length, kilograms for mass, and if we're talking about the motion of something, meters per second. And while this is the agreed upon scientific unit system, you should be aware that other systems do exist, which means things can very easily get very confusing if you forget your units. And you might be thinking, oh come on, who mixes up units? Well, it happens more often than you think. Even rocket scientists have done it. I mean, a Mars orbiter actually crashed due to a mix up in units. No seriously, that actually happened."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "Imagine that I'm standing here holding the end of a rope. I'm over here on the left end, and while holding the rope, I rapidly move my hand up, down, and back to the starting position. If we were to take a snapshot of the rope immediately after I finish my motion, we're going to see something like this. The rope has a squiggly disturbance that mirrors the motion I made with my hand, up, down, and back to the middle. And the rest of the rope is still flat. You might have seen something like this if you've ever played with a jump rope and wiggled it back and forth, or a slinky and you've seen that oscillate back and forth on the ground, or if you've been in the gym and seen somebody doing exercises with large battle ropes, slamming them up and down repeatedly. And we know that over time, this disturbance is actually going to make its way through the rope."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "The rope has a squiggly disturbance that mirrors the motion I made with my hand, up, down, and back to the middle. And the rest of the rope is still flat. You might have seen something like this if you've ever played with a jump rope and wiggled it back and forth, or a slinky and you've seen that oscillate back and forth on the ground, or if you've been in the gym and seen somebody doing exercises with large battle ropes, slamming them up and down repeatedly. And we know that over time, this disturbance is actually going to make its way through the rope. If this is what we observe right after my hand motion, at some later point in time, we will observe that the beginning of the rope is back to its original shape, this squiggly disturbance has made its way further down the rope, and it will keep traveling in this direction until it reaches the end of the rope. This is exactly what a wave is in physics. A wave is a disturbance, in this case, the squiggle in the rope caused by my hand motion, and that disturbance can propagate, it can travel or move in a particular direction."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "And we know that over time, this disturbance is actually going to make its way through the rope. If this is what we observe right after my hand motion, at some later point in time, we will observe that the beginning of the rope is back to its original shape, this squiggly disturbance has made its way further down the rope, and it will keep traveling in this direction until it reaches the end of the rope. This is exactly what a wave is in physics. A wave is a disturbance, in this case, the squiggle in the rope caused by my hand motion, and that disturbance can propagate, it can travel or move in a particular direction. So a wave is a disturbance that can propagate. This particular example is called a mechanical wave. It's called a mechanical wave because the disturbance is traveling through a medium, in this case, the rope."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "A wave is a disturbance, in this case, the squiggle in the rope caused by my hand motion, and that disturbance can propagate, it can travel or move in a particular direction. So a wave is a disturbance that can propagate. This particular example is called a mechanical wave. It's called a mechanical wave because the disturbance is traveling through a medium, in this case, the rope. So mechanical waves travel through a medium. One important point about waves that is worth noting right now is that waves transfer energy without transferring matter. So what that means is that the disturbance that is moving here, this squiggly shape, is moving through the rope, but it isn't moving the rope to a different position."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "It's called a mechanical wave because the disturbance is traveling through a medium, in this case, the rope. So mechanical waves travel through a medium. One important point about waves that is worth noting right now is that waves transfer energy without transferring matter. So what that means is that the disturbance that is moving here, this squiggly shape, is moving through the rope, but it isn't moving the rope to a different position. Any part of the rope might go up and down as a wave travels through that section, but the rope itself is not going anywhere. Rather, it's the kinetic energy imparted to the rope by my hand that is transferring from particle to particle and making its way through the rope. So waves transfer energy but not matter."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "So what that means is that the disturbance that is moving here, this squiggly shape, is moving through the rope, but it isn't moving the rope to a different position. Any part of the rope might go up and down as a wave travels through that section, but the rope itself is not going anywhere. Rather, it's the kinetic energy imparted to the rope by my hand that is transferring from particle to particle and making its way through the rope. So waves transfer energy but not matter. So in my first example, I only jerked my hand up and down once, which created a single wave pulse that moved through my rope. If instead I were to keep moving my hand up and down consistently, I would see a waveform that looks something like this. And when we model a wave, there are a few key characteristics that we need to know about that wave."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "So waves transfer energy but not matter. So in my first example, I only jerked my hand up and down once, which created a single wave pulse that moved through my rope. If instead I were to keep moving my hand up and down consistently, I would see a waveform that looks something like this. And when we model a wave, there are a few key characteristics that we need to know about that wave. First is the period. Period is measured in seconds, and it tells us how long it takes for one wave cycle to complete. Next is the wavelength."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "And when we model a wave, there are a few key characteristics that we need to know about that wave. First is the period. Period is measured in seconds, and it tells us how long it takes for one wave cycle to complete. Next is the wavelength. Measured in units of distance, like meters, the wavelength is the distance between identical points of adjacent waves. And finally, there's frequency. So the waveform that we've drawn here takes one second."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "Next is the wavelength. Measured in units of distance, like meters, the wavelength is the distance between identical points of adjacent waves. And finally, there's frequency. So the waveform that we've drawn here takes one second. There are four cycles in that one second. That means it has a frequency of four Hertz, or four cycles per second. So the frequency, measured in cycles per second, tells us how many wave cycles there are every second."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "So the waveform that we've drawn here takes one second. There are four cycles in that one second. That means it has a frequency of four Hertz, or four cycles per second. So the frequency, measured in cycles per second, tells us how many wave cycles there are every second. Now using just these basic anatomical properties of a wave, we can start to figure out more interesting physical characteristics, like speed or distance over time. If we want to know how fast a wave is traveling, we can take its wavelength, which is the distance covered by a single cycle, and multiply that by the frequency, which is how many cycles are completed in a second, a given amount of time. The cycles cancel out, and we're left with units of distance over time, the same as speed."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "So the frequency, measured in cycles per second, tells us how many wave cycles there are every second. Now using just these basic anatomical properties of a wave, we can start to figure out more interesting physical characteristics, like speed or distance over time. If we want to know how fast a wave is traveling, we can take its wavelength, which is the distance covered by a single cycle, and multiply that by the frequency, which is how many cycles are completed in a second, a given amount of time. The cycles cancel out, and we're left with units of distance over time, the same as speed. And that's our equation for the speed of the wave, wavelength times frequency. The standard units for speed are meters per second. There are a couple factors that can affect the speed of a wave."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "The cycles cancel out, and we're left with units of distance over time, the same as speed. And that's our equation for the speed of the wave, wavelength times frequency. The standard units for speed are meters per second. There are a couple factors that can affect the speed of a wave. The first is the wave type. So different types of waves move at different speeds. A relatable example of different waves moving at different speeds is lightning."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "There are a couple factors that can affect the speed of a wave. The first is the wave type. So different types of waves move at different speeds. A relatable example of different waves moving at different speeds is lightning. If you've ever seen lightning strike or been in a thunderstorm, you know that the first thing you see is the flash of lightning. And then you hear the thunder associated with that lightning flash. So the lightning comes first, and the thunder comes second."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "A relatable example of different waves moving at different speeds is lightning. If you've ever seen lightning strike or been in a thunderstorm, you know that the first thing you see is the flash of lightning. And then you hear the thunder associated with that lightning flash. So the lightning comes first, and the thunder comes second. That's because those are two different waves that are part of the same phenomenon. When the lightning strike hits, you see the flash first because that's an electromagnetic wave, light. It travels much faster than the sound associated with the lightning strike."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "So the lightning comes first, and the thunder comes second. That's because those are two different waves that are part of the same phenomenon. When the lightning strike hits, you see the flash first because that's an electromagnetic wave, light. It travels much faster than the sound associated with the lightning strike. Electromagnetic waves are special not only because they travel really fast, but they also don't need a medium to travel through. The thunder, on the other hand, is a sound wave, traveling slower than the light. So you'll see the lightning before you hear the thunder."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "It travels much faster than the sound associated with the lightning strike. Electromagnetic waves are special not only because they travel really fast, but they also don't need a medium to travel through. The thunder, on the other hand, is a sound wave, traveling slower than the light. So you'll see the lightning before you hear the thunder. Different wave types move at different speeds. The second key factor that can affect the speed of a wave is the medium through which the wave travels. And we'll consider sound as an example here."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "So you'll see the lightning before you hear the thunder. Different wave types move at different speeds. The second key factor that can affect the speed of a wave is the medium through which the wave travels. And we'll consider sound as an example here. So when someone is talking, right, we have this talking head creating some vibrations of the particles in front of their mouth. That's the sound wave. It's the vibration of those particles propagating through the air."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "And we'll consider sound as an example here. So when someone is talking, right, we have this talking head creating some vibrations of the particles in front of their mouth. That's the sound wave. It's the vibration of those particles propagating through the air. When you speak, your vocal cords exert force on the particles just in front of you. They vibrate back and forth, creating a compression that transfers to the surrounding particles. As the vibrations continue to propagate, the sound travels."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "It's the vibration of those particles propagating through the air. When you speak, your vocal cords exert force on the particles just in front of you. They vibrate back and forth, creating a compression that transfers to the surrounding particles. As the vibrations continue to propagate, the sound travels. You can imagine that if these particles are packed closer together, those vibrations are going to transfer a lot more quickly because the particles are colliding much faster than if they're further apart. So sound travels much faster in water, a liquid, than it does in air for that exact reason. The particles in the liquid are closer together."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "As the vibrations continue to propagate, the sound travels. You can imagine that if these particles are packed closer together, those vibrations are going to transfer a lot more quickly because the particles are colliding much faster than if they're further apart. So sound travels much faster in water, a liquid, than it does in air for that exact reason. The particles in the liquid are closer together. Since they're closer compacted, they collide more and the propagation of the wave happens faster. So different waves move at different speeds and the medium through which a wave travels can also affect the speed of a wave. All right, so let's try to summarize all this information."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "The particles in the liquid are closer together. Since they're closer compacted, they collide more and the propagation of the wave happens faster. So different waves move at different speeds and the medium through which a wave travels can also affect the speed of a wave. All right, so let's try to summarize all this information. We have waves, a wave, a disturbance that can propagate, and it has a few key characteristics. There's the period, or how long it takes one cycle to complete. There's the wavelength, the distance between identical points on two waves that are next to each other, and the frequency, which is how many wave cycles complete in one second."}, {"video_title": "Wave properties   Wave properties   High School Physics   Khan Academy.mp3", "Sentence": "All right, so let's try to summarize all this information. We have waves, a wave, a disturbance that can propagate, and it has a few key characteristics. There's the period, or how long it takes one cycle to complete. There's the wavelength, the distance between identical points on two waves that are next to each other, and the frequency, which is how many wave cycles complete in one second. In this case, we have two cycles in one second for a frequency of two Hertz. Wave speed is found by multiplying wavelength and frequency, and that wave speed is affected by the type of wave and the medium through which the wave travels. Mechanical waves are waves that travel through a medium, so sound, a slinky or rope, ocean waves, and electromagnetic waves like light are special because they can travel through a vacuum."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "So I have three different asteroids over here and they have different masses. And we'll talk a lot more about what mass means. But one way to think about it is how much stuff there is there. There's other ways to think about it. And so let's say that this first asteroid is twice the mass of either of these two smaller ones. And these two smaller ones have the same mass. Now we've attached the back of a rocket to each of these asteroids."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "There's other ways to think about it. And so let's say that this first asteroid is twice the mass of either of these two smaller ones. And these two smaller ones have the same mass. Now we've attached the back of a rocket to each of these asteroids. In fact, this one over here has two rockets. And we're gonna assume that all of the rockets are equivalent and we ignite them all. And so they all exert the same force each on the asteroid."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Now we've attached the back of a rocket to each of these asteroids. In fact, this one over here has two rockets. And we're gonna assume that all of the rockets are equivalent and we ignite them all. And so they all exert the same force each on the asteroid. So for example, we have a net force acting leftward on this large asteroid. We have the same net force acting on this smaller asteroid also going to the left. And on this other smaller asteroid, we have two times that net force acting to the left."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And so they all exert the same force each on the asteroid. So for example, we have a net force acting leftward on this large asteroid. We have the same net force acting on this smaller asteroid also going to the left. And on this other smaller asteroid, we have two times that net force acting to the left. So what I want you to do is pause this video and think about which of these asteroids is going to be accelerated the most and which of these asteroids is going to be accelerated the least. All right, so you might have an intuition that the larger the force, the more acceleration you might see. So let me write it like this."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And on this other smaller asteroid, we have two times that net force acting to the left. So what I want you to do is pause this video and think about which of these asteroids is going to be accelerated the most and which of these asteroids is going to be accelerated the least. All right, so you might have an intuition that the larger the force, the more acceleration you might see. So let me write it like this. So you might get a sense that if you increase your force, that that's also going to increase your acceleration. And it does turn out that that is indeed the case. Now, the other notion that you might have is that the more of the stuff that there is, the more mass that you have, the harder it is to accelerate it."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "So let me write it like this. So you might get a sense that if you increase your force, that that's also going to increase your acceleration. And it does turn out that that is indeed the case. Now, the other notion that you might have is that the more of the stuff that there is, the more mass that you have, the harder it is to accelerate it. So if your mass is larger, then your acceleration is lower. And it turns out that these things are all proportional. So for example, if we just compare these two masses right over here, they have the same net force acting on it."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Now, the other notion that you might have is that the more of the stuff that there is, the more mass that you have, the harder it is to accelerate it. So if your mass is larger, then your acceleration is lower. And it turns out that these things are all proportional. So for example, if we just compare these two masses right over here, they have the same net force acting on it. And I keep saying net force, that means you just net out all of the forces acting in a certain dimension. For example, if I had another identical rocket acting in the opposite direction, they would net out. And this asteroid right over here wouldn't be accelerated at all."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "So for example, if we just compare these two masses right over here, they have the same net force acting on it. And I keep saying net force, that means you just net out all of the forces acting in a certain dimension. For example, if I had another identical rocket acting in the opposite direction, they would net out. And this asteroid right over here wouldn't be accelerated at all. But going back to our example here, we have the same net force acting on each of these asteroids, but the first asteroid has twice the mass of the second asteroid. So how do you think the accelerations will relate? Well, as you might imagine, the acceleration on the larger asteroid is going to be half the acceleration on this asteroid."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And this asteroid right over here wouldn't be accelerated at all. But going back to our example here, we have the same net force acting on each of these asteroids, but the first asteroid has twice the mass of the second asteroid. So how do you think the accelerations will relate? Well, as you might imagine, the acceleration on the larger asteroid is going to be half the acceleration on this asteroid. Or another way to think about it, this asteroid is going to have twice the acceleration as this first asteroid. And that's because it has half the mass. And one way you can relate force, mass, and acceleration, and this is one of the most important equations in all of physics, is that force is going to be equal to mass times acceleration."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "Well, as you might imagine, the acceleration on the larger asteroid is going to be half the acceleration on this asteroid. Or another way to think about it, this asteroid is going to have twice the acceleration as this first asteroid. And that's because it has half the mass. And one way you can relate force, mass, and acceleration, and this is one of the most important equations in all of physics, is that force is going to be equal to mass times acceleration. Or I could say the magnitude of the force is equal to the mass times the magnitude of the acceleration. So notice, in this example right over here, our forces are the same, but the masses are different. If I have half the masses I have over here, I'm going to have twice the acceleration."}, {"video_title": "Force, mass and acceleration   Movement and forces   Middle school physics   Khan Academy.mp3", "Sentence": "And one way you can relate force, mass, and acceleration, and this is one of the most important equations in all of physics, is that force is going to be equal to mass times acceleration. Or I could say the magnitude of the force is equal to the mass times the magnitude of the acceleration. So notice, in this example right over here, our forces are the same, but the masses are different. If I have half the masses I have over here, I'm going to have twice the acceleration. And that might make intuitive sense if you've ever tried to apply the same force to something that has a small mass versus something that has a large mass. Now, if we compare these two asteroids, they have the same mass here, but the force here, the net force acting in that left direction is double. So if you double the force, don't change the mass, well then you're going to have twice the acceleration."}, {"video_title": "Introduction to Middle school physics   Khan Academy.mp3", "Sentence": "Hi everyone, Sal Khan here, and welcome to Middle School Physics. I have Iman Howard, who manages all of our STEM content. Iman, why should folks be excited about middle school physics? So, middle school physics is like the only science out there that explains how things happen. And so basically, everything's made of matter. Me, you, the chair that I'm sitting on, and this course is gonna explore how we exist in the natural world. So for example, we talk a little bit about movement and forces, and we learned that everything, everything that we have a collision with has this equal but opposite force that's applied when the collision happens."}, {"video_title": "Introduction to Middle school physics   Khan Academy.mp3", "Sentence": "So, middle school physics is like the only science out there that explains how things happen. And so basically, everything's made of matter. Me, you, the chair that I'm sitting on, and this course is gonna explore how we exist in the natural world. So for example, we talk a little bit about movement and forces, and we learned that everything, everything that we have a collision with has this equal but opposite force that's applied when the collision happens. So that's why when you give those high fives and then your hand starts stinging, it's because the same force you gave your buddy is the same force they gave you back. And then we also talk about force in a way where it doesn't touch you. So like, I'm thinking like Star Wars, there's like this force energy, like gravitational, there's magnetic energy, there's electric energy."}, {"video_title": "Introduction to Middle school physics   Khan Academy.mp3", "Sentence": "So for example, we talk a little bit about movement and forces, and we learned that everything, everything that we have a collision with has this equal but opposite force that's applied when the collision happens. So that's why when you give those high fives and then your hand starts stinging, it's because the same force you gave your buddy is the same force they gave you back. And then we also talk about force in a way where it doesn't touch you. So like, I'm thinking like Star Wars, there's like this force energy, like gravitational, there's magnetic energy, there's electric energy. And then finally, we get into waves. And we talk a little bit about how waves exist, whether it's sound waves or even the waves in the ocean. What do you think's exciting about it?"}, {"video_title": "Introduction to Middle school physics   Khan Academy.mp3", "Sentence": "So like, I'm thinking like Star Wars, there's like this force energy, like gravitational, there's magnetic energy, there's electric energy. And then finally, we get into waves. And we talk a little bit about how waves exist, whether it's sound waves or even the waves in the ocean. What do you think's exciting about it? Oh, well, that's a dangerous question to ask me. I wanted to be a physicist and I still aspire to be it because we kind of wake up in this cosmos and we're just trying to understand where we fit in. And physics asks the most fundamental questions about how the universe works."}, {"video_title": "Introduction to Middle school physics   Khan Academy.mp3", "Sentence": "What do you think's exciting about it? Oh, well, that's a dangerous question to ask me. I wanted to be a physicist and I still aspire to be it because we kind of wake up in this cosmos and we're just trying to understand where we fit in. And physics asks the most fundamental questions about how the universe works. And so when I first learned about Newton's laws and fields and all of the things that you just touched on, it started to give me goosebumps because I'm like, wow, we can finally understand how the universe fits together and then use that to make predictions and then think about things that we don't understand. And there is so much that we don't understand. So I think this is the beginning of a very, very exciting journey in physics."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "One type of temporary magnet is called an electromagnet. So what is electromagnetism? Well, the hint is in the name itself. Electro for electrical and magnet for, well, magnet. Let's take a moment to look at the definition of what an electromagnet is. Electromagnets are materials that become magnets in the presence of electricity. But how does that even happen?"}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Electro for electrical and magnet for, well, magnet. Let's take a moment to look at the definition of what an electromagnet is. Electromagnets are materials that become magnets in the presence of electricity. But how does that even happen? Well, it turns out that electrically charged particles in motion actually have small magnetic fields around them. So if we run electricity through a wire, a magnetic field will be created around the wire. Now we can control the strength of this magnetic field in a couple of ways."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "But how does that even happen? Well, it turns out that electrically charged particles in motion actually have small magnetic fields around them. So if we run electricity through a wire, a magnetic field will be created around the wire. Now we can control the strength of this magnetic field in a couple of ways. We can move more electric charges through the wire at a faster rate, and we do this by increasing the electrical current. The second way is to increase the density of the charged particles, and we can do this by looping the wire into a coil. This gives us more charged particles with magnetic fields in a small space, strengthening the magnetic force."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Now we can control the strength of this magnetic field in a couple of ways. We can move more electric charges through the wire at a faster rate, and we do this by increasing the electrical current. The second way is to increase the density of the charged particles, and we can do this by looping the wire into a coil. This gives us more charged particles with magnetic fields in a small space, strengthening the magnetic force. The other way we can control electromagnets is the direction of the magnetic field, and we can do this by changing the direction of the electricity. So if we go back to this wire example from earlier and change the direction of the electricity running through that wire, well, the magnetic fields will also change direction. This makes electromagnets quite different from permanent magnets."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "This gives us more charged particles with magnetic fields in a small space, strengthening the magnetic force. The other way we can control electromagnets is the direction of the magnetic field, and we can do this by changing the direction of the electricity. So if we go back to this wire example from earlier and change the direction of the electricity running through that wire, well, the magnetic fields will also change direction. This makes electromagnets quite different from permanent magnets. So let's take a look at that and compare permanent magnets to electromagnets. Electromagnets are typically made of loops of wire in a coil. The wire is typically made of metal like copper and wrapped around pieces of metal like iron, nickel, or cobalt."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "This makes electromagnets quite different from permanent magnets. So let's take a look at that and compare permanent magnets to electromagnets. Electromagnets are typically made of loops of wire in a coil. The wire is typically made of metal like copper and wrapped around pieces of metal like iron, nickel, or cobalt. This is different from a permanent magnet because permanent magnets don't need this wire. Permanent magnets also have fixed poles. You can't change the north and south poles on these magnets, but as we now know, for electromagnets we can change these poles by changing the direction of the electrical current."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "The wire is typically made of metal like copper and wrapped around pieces of metal like iron, nickel, or cobalt. This is different from a permanent magnet because permanent magnets don't need this wire. Permanent magnets also have fixed poles. You can't change the north and south poles on these magnets, but as we now know, for electromagnets we can change these poles by changing the direction of the electrical current. So if we have an electromagnet with a north and south pole that looks like this and a current flowing in this direction, well, we can change the poles and the direction of the current. Permanent magnets have a fixed strength, so they can't change the direction of the current. Electromagnets have a fixed strength, but we just talked about how we can change the strength of electromagnets, so electromagnets have adjustable strength."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "You can't change the north and south poles on these magnets, but as we now know, for electromagnets we can change these poles by changing the direction of the electrical current. So if we have an electromagnet with a north and south pole that looks like this and a current flowing in this direction, well, we can change the poles and the direction of the current. Permanent magnets have a fixed strength, so they can't change the direction of the current. Electromagnets have a fixed strength, but we just talked about how we can change the strength of electromagnets, so electromagnets have adjustable strength. And finally, electromagnets need a power source in order to generate the electricity required to produce magnetic fields. Permanent magnets do not need a power source, but this means that we can also turn electromagnets on and off, which is pretty cool when you think about it. On the other hand, electromagnets can also be turned on and off."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Electromagnets have a fixed strength, but we just talked about how we can change the strength of electromagnets, so electromagnets have adjustable strength. And finally, electromagnets need a power source in order to generate the electricity required to produce magnetic fields. Permanent magnets do not need a power source, but this means that we can also turn electromagnets on and off, which is pretty cool when you think about it. On the other hand, electromagnets can also be turned on and off. So if we have a magnet that's spinning, we can turn it on and off. Now you might be thinking, if electrical charge can affect magnetism, can magnetism affect electrical charge? Absolutely!"}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "On the other hand, electromagnets can also be turned on and off. So if we have a magnet that's spinning, we can turn it on and off. Now you might be thinking, if electrical charge can affect magnetism, can magnetism affect electrical charge? Absolutely! Let's look at how we can do that. The only way to do this is by changing the magnetic field around the charged particles. In fact, spinning magnets is how most of the electricity we use in cities and homes is generated."}, {"video_title": "Electromagnetism   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Absolutely! Let's look at how we can do that. The only way to do this is by changing the magnetic field around the charged particles. In fact, spinning magnets is how most of the electricity we use in cities and homes is generated. A turbine spins a magnet inside a coil to produce electricity, and since electromagnets need a power source, this turbine is powered by wind. So you can see why electromagnetism is an incredibly important force, and this isn't the only important application of it. We use electromagnets in all sorts of other applications, from motors to speakers, and even medical scanners."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Nothing is touching it once you let it go. How can there be a force on it? Well, this is because Earth's gravitational force is pulling the ball, and gravity is a non-contact force. Non-contact forces don't have to touch an object to exert a force on it. Instead, these forces act over a region. So, if an object is in that region, it will be affected by the force. In this case, the ball is in Earth's gravitational field, and so it feels an attractive force towards the Earth, and the ball falls to the ground."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Non-contact forces don't have to touch an object to exert a force on it. Instead, these forces act over a region. So, if an object is in that region, it will be affected by the force. In this case, the ball is in Earth's gravitational field, and so it feels an attractive force towards the Earth, and the ball falls to the ground. Field forces include non-contact forces such as electric, magnetic, and of course, gravitational forces. So, since these forces are non-contact, they can exert a force on objects they aren't touching, but how do these objects know if there's a force between them? To explain these non-contact forces, scientists eventually developed the idea that these objects were surrounded by something called a field."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "In this case, the ball is in Earth's gravitational field, and so it feels an attractive force towards the Earth, and the ball falls to the ground. Field forces include non-contact forces such as electric, magnetic, and of course, gravitational forces. So, since these forces are non-contact, they can exert a force on objects they aren't touching, but how do these objects know if there's a force between them? To explain these non-contact forces, scientists eventually developed the idea that these objects were surrounded by something called a field. So what is a field? A field extends through space from an object with certain physical properties. What are those?"}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "To explain these non-contact forces, scientists eventually developed the idea that these objects were surrounded by something called a field. So what is a field? A field extends through space from an object with certain physical properties. What are those? Well, for gravitational forces, these affect objects with mass. So, any object with mass has a gravitational field surrounding it that points towards the object's center. The further you move away from the object, the less dense the field, and weaker the field becomes."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "What are those? Well, for gravitational forces, these affect objects with mass. So, any object with mass has a gravitational field surrounding it that points towards the object's center. The further you move away from the object, the less dense the field, and weaker the field becomes. Electric forces affect charged objects. So, an electric field surrounds any object with a net charge, and the direction of this field will depend on the charge. Magnetic fields will affect magnets and any other material with magnetic properties."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "The further you move away from the object, the less dense the field, and weaker the field becomes. Electric forces affect charged objects. So, an electric field surrounds any object with a net charge, and the direction of this field will depend on the charge. Magnetic fields will affect magnets and any other material with magnetic properties. Each spot on a field has two things associated with it. Magnitude and direction. And these help us predict what forces objects will experience when they're in the field."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Magnetic fields will affect magnets and any other material with magnetic properties. Each spot on a field has two things associated with it. Magnitude and direction. And these help us predict what forces objects will experience when they're in the field. So, let's look at an example to help understand this. Say we have a planet. Now, the planet has a lot of mass, so we know it's going to be surrounded by a gravitational field that points towards the center of the planet."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "And these help us predict what forces objects will experience when they're in the field. So, let's look at an example to help understand this. Say we have a planet. Now, the planet has a lot of mass, so we know it's going to be surrounded by a gravitational field that points towards the center of the planet. I can draw these little field lines that show the direction of the field and its strength. As we move away from the planet, the field will start to weaken, and I'm going to represent that by a less dense field with these arrows. Now, let's say there's an asteroid moving near the planet in this direction."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Now, the planet has a lot of mass, so we know it's going to be surrounded by a gravitational field that points towards the center of the planet. I can draw these little field lines that show the direction of the field and its strength. As we move away from the planet, the field will start to weaken, and I'm going to represent that by a less dense field with these arrows. Now, let's say there's an asteroid moving near the planet in this direction. I know that the asteroid, as it's shown here, is in the outskirts of this planet's gravitational field, so it is going to feel some gravitational attraction towards the planet, which we can draw with this vector FG, which is force of gravity. Now, because it's attracted to the planet, the asteroid will continue to move towards the planet. And the closer the asteroid gets to the planet, the stronger the field and the stronger the force of attraction it will feel."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Now, let's say there's an asteroid moving near the planet in this direction. I know that the asteroid, as it's shown here, is in the outskirts of this planet's gravitational field, so it is going to feel some gravitational attraction towards the planet, which we can draw with this vector FG, which is force of gravity. Now, because it's attracted to the planet, the asteroid will continue to move towards the planet. And the closer the asteroid gets to the planet, the stronger the field and the stronger the force of attraction it will feel. And so, in this way, scientists can use fields to help predict behavior of objects experiencing non-contact forces. And all of this may sound kind of odd, but you probably already think about forces this way. For example, if we go back to the ball that you know is going to fall, you knew this because the force of gravity from Earth was going to pull the ball towards the Earth."}, {"video_title": "Fields   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "And the closer the asteroid gets to the planet, the stronger the field and the stronger the force of attraction it will feel. And so, in this way, scientists can use fields to help predict behavior of objects experiencing non-contact forces. And all of this may sound kind of odd, but you probably already think about forces this way. For example, if we go back to the ball that you know is going to fall, you knew this because the force of gravity from Earth was going to pull the ball towards the Earth. But now you also know that that's because Earth's gravity is a field force. And so the ball is in the field of gravity for Earth and experiences an attractive gravitational force. So, while fields may sound mysterious, they really just mean that a force is felt over a distance."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Magnets are these neat objects that are able to attract metals like iron. Magnets are used in all sorts of things from holding paper on your refrigerator to computers to compasses. So magnets can be used to stick things together, point us in the right direction, and even lift things. And they do this through magnetic forces. If you've handled two magnets, you've felt magnetic forces, even when the magnets weren't touching each other. That's because magnetic forces are non-contact forces, which just means they can affect other objects they aren't even touching. Magnets will attract or repel each other and this attraction or repulsion is a magnetic force."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "And they do this through magnetic forces. If you've handled two magnets, you've felt magnetic forces, even when the magnets weren't touching each other. That's because magnetic forces are non-contact forces, which just means they can affect other objects they aren't even touching. Magnets will attract or repel each other and this attraction or repulsion is a magnetic force. But magnetic forces don't affect everything the same way. Otherwise a magnet would stick to you, not just a refrigerator. In this video, we're going to talk about the magnetic forces between two magnets."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Magnets will attract or repel each other and this attraction or repulsion is a magnetic force. But magnetic forces don't affect everything the same way. Otherwise a magnet would stick to you, not just a refrigerator. In this video, we're going to talk about the magnetic forces between two magnets. So why do magnets sometimes attract each other and other times repel each other? Well, this has to do with the orientation of the magnets. Orientation is really just a fancy word for how the magnets are positioned compared to one another."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "In this video, we're going to talk about the magnetic forces between two magnets. So why do magnets sometimes attract each other and other times repel each other? Well, this has to do with the orientation of the magnets. Orientation is really just a fancy word for how the magnets are positioned compared to one another. You see, it turns out that each magnet has a north and a south pole. But what does this have to do with attraction or repulsion? Well, as you may have heard, opposites attract."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Orientation is really just a fancy word for how the magnets are positioned compared to one another. You see, it turns out that each magnet has a north and a south pole. But what does this have to do with attraction or repulsion? Well, as you may have heard, opposites attract. So if you face the north pole of one magnet to the south pole of another magnet, guess what? They will be attracted to each other. But if you turn one of those magnets around so that you have two north poles facing each other, they will repel."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Well, as you may have heard, opposites attract. So if you face the north pole of one magnet to the south pole of another magnet, guess what? They will be attracted to each other. But if you turn one of those magnets around so that you have two north poles facing each other, they will repel. And the same thing would happen if it was two south poles facing each other. So the direction of the magnetic force completely depends on the orientation of the magnets. Orientation though is just one thing that affects magnetic forces."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "But if you turn one of those magnets around so that you have two north poles facing each other, they will repel. And the same thing would happen if it was two south poles facing each other. So the direction of the magnetic force completely depends on the orientation of the magnets. Orientation though is just one thing that affects magnetic forces. The strength of magnetic forces depends on a couple of things. For one, distance. If you've ever held two magnets, you may have noticed that when you move them closer, they seem to almost jump together."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Orientation though is just one thing that affects magnetic forces. The strength of magnetic forces depends on a couple of things. For one, distance. If you've ever held two magnets, you may have noticed that when you move them closer, they seem to almost jump together. Or if you try to push two like poles together, they get harder and harder to hold together the closer you get. This is because magnetic forces depend on distance. The closer the two magnets are together, the stronger the force between them."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "If you've ever held two magnets, you may have noticed that when you move them closer, they seem to almost jump together. Or if you try to push two like poles together, they get harder and harder to hold together the closer you get. This is because magnetic forces depend on distance. The closer the two magnets are together, the stronger the force between them. So as the distance decreases, the force increases. But the farther away they are, the weaker the magnetic force is. So distance increases, force decreases."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "The closer the two magnets are together, the stronger the force between them. So as the distance decreases, the force increases. But the farther away they are, the weaker the magnetic force is. So distance increases, force decreases. The other big factor that affects how strong a magnetic force is, well, the magnets themselves. Some magnets are really weak, like a lot of refrigerator magnets. Others are so strong that even tiny ones can be almost impossible to pull apart."}, {"video_title": "Magnetic forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "So distance increases, force decreases. The other big factor that affects how strong a magnetic force is, well, the magnets themselves. Some magnets are really weak, like a lot of refrigerator magnets. Others are so strong that even tiny ones can be almost impossible to pull apart. Some of these stronger magnets are even used to make high speed trains levitate off the ground. And yes, this rectangle is supposed to be a train. While my drawing isn't amazing, the fact that magnetic forces can levitate a train is."}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Any massive object that is in motion then has kinetic energy, but how much? First, let's consider some comparisons. This nice rat family, papa, mama, brother, and sister are sitting down to dinner at a long table passing blocks of cheese back and forth. Papa rat asks for the cheddar cheese and there are two identical blocks. Brother rat pushes one and sister rat pushes the other so that the second cheese is traveling twice as fast as the first cheese. Which piece of cheddar cheese do you think has more kinetic energy? Yes, it's the one going faster."}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Papa rat asks for the cheddar cheese and there are two identical blocks. Brother rat pushes one and sister rat pushes the other so that the second cheese is traveling twice as fast as the first cheese. Which piece of cheddar cheese do you think has more kinetic energy? Yes, it's the one going faster. Now papa rat doesn't need both pieces of cheddar so he eats one and sends one back along with a small piece of swiss that weighs half as much as the piece of cheddar. Papa rat has better manners than his children so he sends them both back at the same speed. Which piece of cheese would you think has more kinetic energy now?"}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Yes, it's the one going faster. Now papa rat doesn't need both pieces of cheddar so he eats one and sends one back along with a small piece of swiss that weighs half as much as the piece of cheddar. Papa rat has better manners than his children so he sends them both back at the same speed. Which piece of cheese would you think has more kinetic energy now? Yes, the heavier or more massive object, in this case the cheddar, will have more kinetic energy. Let's make it a little more complicated. Brother and sister rat are full so they send the cheeses back for mama rat."}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Which piece of cheese would you think has more kinetic energy now? Yes, the heavier or more massive object, in this case the cheddar, will have more kinetic energy. Let's make it a little more complicated. Brother and sister rat are full so they send the cheeses back for mama rat. Brother rat pushes the larger piece of cheddar and sister rat pushes the smaller piece of swiss so that the swiss is going twice as fast as the cheddar. Now which cheese has more kinetic energy? In fact, it turns out that it's the swiss in this scenario."}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "Brother and sister rat are full so they send the cheeses back for mama rat. Brother rat pushes the larger piece of cheddar and sister rat pushes the smaller piece of swiss so that the swiss is going twice as fast as the cheddar. Now which cheese has more kinetic energy? In fact, it turns out that it's the swiss in this scenario. Kinetic energy depends on both mass and speed but the dependence on speed is stronger. This estimation of kinetic energy can be quantified in an equation that lets us calculate kinetic energy exactly. We said kinetic energy depends on the mass and the speed which we'll write as v for velocity so we can start with ke equals m times v. But we said that it depends more on the speed so the velocity here is actually squared."}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "In fact, it turns out that it's the swiss in this scenario. Kinetic energy depends on both mass and speed but the dependence on speed is stronger. This estimation of kinetic energy can be quantified in an equation that lets us calculate kinetic energy exactly. We said kinetic energy depends on the mass and the speed which we'll write as v for velocity so we can start with ke equals m times v. But we said that it depends more on the speed so the velocity here is actually squared. This means that if an object's mass doubles its kinetic energy also doubles but if its speed doubles the kinetic energy actually quadruples. And there's also a constant factor of one half at the beginning of the equation but we won't go into the details of the math of deriving this today. So this is the equation for kinetic energy one half m v squared."}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "We said kinetic energy depends on the mass and the speed which we'll write as v for velocity so we can start with ke equals m times v. But we said that it depends more on the speed so the velocity here is actually squared. This means that if an object's mass doubles its kinetic energy also doubles but if its speed doubles the kinetic energy actually quadruples. And there's also a constant factor of one half at the beginning of the equation but we won't go into the details of the math of deriving this today. So this is the equation for kinetic energy one half m v squared. Let's apply this equation to our cheesy example. Say the swiss has a mass of 0.05 kilograms which makes the cheddar's mass 0.1 kilograms. When both cheeses have the same speed say two meters per second the cheddar's kinetic energy is one half times 0.1 kilograms times two meters per second squared which is 0.2 joules."}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "So this is the equation for kinetic energy one half m v squared. Let's apply this equation to our cheesy example. Say the swiss has a mass of 0.05 kilograms which makes the cheddar's mass 0.1 kilograms. When both cheeses have the same speed say two meters per second the cheddar's kinetic energy is one half times 0.1 kilograms times two meters per second squared which is 0.2 joules. The swiss's kinetic energy is one half times 0.05 kilograms times two meters per second squared which is 0.1 joules or half the kinetic energy of the cheddar. So we can see that at the same speed the cheddar has more kinetic energy because it has more mass. But when the swiss has a speed of four meters per second and the cheddar still has a speed of two meters per second the swiss's kinetic energy is now one half times 0.05 kilograms times four meters per second squared which is 0.4 joules."}, {"video_title": "Kinetic energy   Energy   Middle school physics   Khan Academy.mp3", "Sentence": "When both cheeses have the same speed say two meters per second the cheddar's kinetic energy is one half times 0.1 kilograms times two meters per second squared which is 0.2 joules. The swiss's kinetic energy is one half times 0.05 kilograms times two meters per second squared which is 0.1 joules or half the kinetic energy of the cheddar. So we can see that at the same speed the cheddar has more kinetic energy because it has more mass. But when the swiss has a speed of four meters per second and the cheddar still has a speed of two meters per second the swiss's kinetic energy is now one half times 0.05 kilograms times four meters per second squared which is 0.4 joules. So now the kinetic energy of the swiss is twice the kinetic energy of the cheddar so we can see that even though the cheddar has more mass the swiss has more kinetic energy because it's going faster. In summary kinetic energy is the motion energy of an object. The equation for kinetic energy is one half m v squared so as mass increases kinetic energy increases like the more massive cheddar versus the swiss and as velocity increases kinetic energy increases even more like the speedy swiss versus the slower cheddar."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "For example, here we have the sun, 93 million miles away on average. And imagine the different materials that the light has to travel through from the sun to say hit one of these sand particles right over here. Think about what it needs to be transmitted through. Well, it's going to travel through 93 million miles of the vacuum of space. And that's one of the amazing things about light waves is that they don't need a medium. They can travel through vacuum, through emptiness. But then it's going to travel through several miles of Earth's atmosphere."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Well, it's going to travel through 93 million miles of the vacuum of space. And that's one of the amazing things about light waves is that they don't need a medium. They can travel through vacuum, through emptiness. But then it's going to travel through several miles of Earth's atmosphere. So it's going to travel through several miles of Earth's atmosphere. It'll hit the lenses of these sunglasses. It'll actually travel through the lenses of the sunglass."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "But then it's going to travel through several miles of Earth's atmosphere. So it's going to travel through several miles of Earth's atmosphere. It'll hit the lenses of these sunglasses. It'll actually travel through the lenses of the sunglass. The sunglass has some width or some depth to it. And then it'll go out onto the other side and it will hit the sand right over here. Now, one thing you might realize is the amount of transmission and what gets transmitted is dependent on the wavelengths of the wave, in this case, the wavelengths of light, and also about the material that they are going through."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "It'll actually travel through the lenses of the sunglass. The sunglass has some width or some depth to it. And then it'll go out onto the other side and it will hit the sand right over here. Now, one thing you might realize is the amount of transmission and what gets transmitted is dependent on the wavelengths of the wave, in this case, the wavelengths of light, and also about the material that they are going through. So for example, these sunglasses right over here, many sunglasses try to keep out UV light, ultraviolet light, which is a higher frequency than visible light, but that's what causes sunburns and that can also damage your eyes. So those high frequencies are not making it through. And we could also see that this sunglass right over here, it kind of has an orangish color, which means that things that are closer to that end of the spectrum, closer to the red, the oranges, and the yellows are getting through, which means that it's filtering out blue light."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Now, one thing you might realize is the amount of transmission and what gets transmitted is dependent on the wavelengths of the wave, in this case, the wavelengths of light, and also about the material that they are going through. So for example, these sunglasses right over here, many sunglasses try to keep out UV light, ultraviolet light, which is a higher frequency than visible light, but that's what causes sunburns and that can also damage your eyes. So those high frequencies are not making it through. And we could also see that this sunglass right over here, it kind of has an orangish color, which means that things that are closer to that end of the spectrum, closer to the red, the oranges, and the yellows are getting through, which means that it's filtering out blue light. So the blue light isn't getting transmitted through as much as say the red, orange, and yellow light. And that's why we see this as red, orange, or yellow. And then of course, the light will get to that sand particle."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "And we could also see that this sunglass right over here, it kind of has an orangish color, which means that things that are closer to that end of the spectrum, closer to the red, the oranges, and the yellows are getting through, which means that it's filtering out blue light. So the blue light isn't getting transmitted through as much as say the red, orange, and yellow light. And that's why we see this as red, orange, or yellow. And then of course, the light will get to that sand particle. Now, transmission, as I mentioned, it isn't just about light waves. We could talk about one of our other favorite types of waves, for example, sound waves. If you are in a room, you have probably experienced the fact that even if you were to close the door, and I do this a lot, because I record a lot of videos, this is me in my little closet recording a video, this is a top view for what I'm doing right now."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "And then of course, the light will get to that sand particle. Now, transmission, as I mentioned, it isn't just about light waves. We could talk about one of our other favorite types of waves, for example, sound waves. If you are in a room, you have probably experienced the fact that even if you were to close the door, and I do this a lot, because I record a lot of videos, this is me in my little closet recording a video, this is a top view for what I'm doing right now. A lot of times my kids are in other parts of the house and they're making a lot of noise. And as we've talked about, sound waves are nothing but traveling pressure waves through the air. Those air particles are knocking one into another."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "If you are in a room, you have probably experienced the fact that even if you were to close the door, and I do this a lot, because I record a lot of videos, this is me in my little closet recording a video, this is a top view for what I'm doing right now. A lot of times my kids are in other parts of the house and they're making a lot of noise. And as we've talked about, sound waves are nothing but traveling pressure waves through the air. Those air particles are knocking one into another. But in order to make it to me, they need to get through that wall. And the way they do that is they get transmitted to through that wall. So those air particles make the particles or make the atoms or the molecules in the wall start vibrating."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Those air particles are knocking one into another. But in order to make it to me, they need to get through that wall. And the way they do that is they get transmitted to through that wall. So those air particles make the particles or make the atoms or the molecules in the wall start vibrating. They bump into each other. And then the particles on the other side of the wall will bump into the air in my little closet. And then we will have once again, the sound waves make it to me."}, {"video_title": "Wave transmission   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "So those air particles make the particles or make the atoms or the molecules in the wall start vibrating. They bump into each other. And then the particles on the other side of the wall will bump into the air in my little closet. And then we will have once again, the sound waves make it to me. Now the overall magnitude of the sound, the volume of the sound will likely be diminished. And not all of the frequencies of the sound will be transmitted equally. Different frequencies of sound waves are better at traveling through certain materials, just as we talked about with light waves."}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "If you have, you might have noticed that once you pulled the sock off, it was still attracted to the shirt, even when they weren't touching. What is even happening here? Well, it turns out there's an electric force between the shirt and the sock. Electric forces are a type of non-contact force, which means they can act on objects that aren't even touching. If you've ever noticed two balloons repelling each other, or if you've ever noticed your hair sticking to something like a balloon or a sweater, that's what I'm talking about. But why is there an electric force between the shirt and the sock after they're taken out of the dryer? Well, it turns out that electric forces are caused by a property of matter called electric charge."}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Electric forces are a type of non-contact force, which means they can act on objects that aren't even touching. If you've ever noticed two balloons repelling each other, or if you've ever noticed your hair sticking to something like a balloon or a sweater, that's what I'm talking about. But why is there an electric force between the shirt and the sock after they're taken out of the dryer? Well, it turns out that electric forces are caused by a property of matter called electric charge. Matter is made up of tiny particles that can have positive, negative, or neutral charge. Neutral just means that the electric charge is zero, not positive or negative. When you add up all these charges, most objects tend to have a net charge that is about neutral."}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Well, it turns out that electric forces are caused by a property of matter called electric charge. Matter is made up of tiny particles that can have positive, negative, or neutral charge. Neutral just means that the electric charge is zero, not positive or negative. When you add up all these charges, most objects tend to have a net charge that is about neutral. Otherwise, we'd be attracted to all sorts of things, just like that sock. However, an object's charge can change. So in the dryer, all that heat and movement allowed some negative charges from the shirt to move to the sock."}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "When you add up all these charges, most objects tend to have a net charge that is about neutral. Otherwise, we'd be attracted to all sorts of things, just like that sock. However, an object's charge can change. So in the dryer, all that heat and movement allowed some negative charges from the shirt to move to the sock. Now when you try to separate the two, they are both electrically charged, and there is an electric force between them. So now that we know what causes an electric force, let's look at what affects its direction and strength. An electric force can attract or repel an object."}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "So in the dryer, all that heat and movement allowed some negative charges from the shirt to move to the sock. Now when you try to separate the two, they are both electrically charged, and there is an electric force between them. So now that we know what causes an electric force, let's look at what affects its direction and strength. An electric force can attract or repel an object. But how do you know if an electric force will be attractive or repulsive? Well, as the saying goes, opposites attract. An object with a negative electric charge will be attracted to a positively charged object."}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "An electric force can attract or repel an object. But how do you know if an electric force will be attractive or repulsive? Well, as the saying goes, opposites attract. An object with a negative electric charge will be attracted to a positively charged object. If the second object is also negatively charged, well, the two objects will experience a repulsive force and be repelled from each other. So in order to have the sock and the shirt attracted to each other, they must have opposite net charges because they are experiencing an attractive electric force. What about the strength of that electric force?"}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "An object with a negative electric charge will be attracted to a positively charged object. If the second object is also negatively charged, well, the two objects will experience a repulsive force and be repelled from each other. So in order to have the sock and the shirt attracted to each other, they must have opposite net charges because they are experiencing an attractive electric force. What about the strength of that electric force? Strength will depend on a couple of factors. First, the charge of each object is proportional to the force. The stronger the charges, the stronger the electric force."}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "What about the strength of that electric force? Strength will depend on a couple of factors. First, the charge of each object is proportional to the force. The stronger the charges, the stronger the electric force. So the greater the electric charge, the greater the electric force. Another factor, how far apart the objects are from each other. The electric force will weaken as the distance increases."}, {"video_title": "Electric forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "The stronger the charges, the stronger the electric force. So the greater the electric charge, the greater the electric force. Another factor, how far apart the objects are from each other. The electric force will weaken as the distance increases. So the force decreases as our distance increases. As you move the sock away from the shirt, eventually the attraction between them is so weak that we don't even notice it. So the next time you find yourself doing laundry and having to pull a sock off of a shirt, just remember, invisible electric forces are to blame."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "But focusing on light for a second, we've already talked about how if we go from one medium to another, if we're transmitting from one medium to another, that the light can go through a medium, and as it goes to that boundary from one medium to another, its direction can change. And we talk about that in another video, and we call that refraction. We have refraction when we enter the material, and then we have more refraction when we get out. Now there's other things that light can also do, and that's the focus of this video, reflection and absorption. Now you probably have a sense of what happens with reflection. We can see a reflection of the mountains in the lake right over here. And the reason why we can see the reflection of the mountains in the lake here is because light that is coming from this mountain is hitting the lake, and then it is bouncing off of it, and then coming to an observer's eyeball right over here."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Now there's other things that light can also do, and that's the focus of this video, reflection and absorption. Now you probably have a sense of what happens with reflection. We can see a reflection of the mountains in the lake right over here. And the reason why we can see the reflection of the mountains in the lake here is because light that is coming from this mountain is hitting the lake, and then it is bouncing off of it, and then coming to an observer's eyeball right over here. And so they see the light that's coming from here, they see it as coming from over here because it is bounced off. And so reflection is exactly that. Light is coming in, it hits that other material, and then if it bounces off, that's reflection."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "And the reason why we can see the reflection of the mountains in the lake here is because light that is coming from this mountain is hitting the lake, and then it is bouncing off of it, and then coming to an observer's eyeball right over here. And so they see the light that's coming from here, they see it as coming from over here because it is bounced off. And so reflection is exactly that. Light is coming in, it hits that other material, and then if it bounces off, that's reflection. Now the other thing that light can do is get absorbed. Absorbed is when the material doesn't reflect any and doesn't transmit any through it, if it's completely absorbing. And so that would be a situation like this."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Light is coming in, it hits that other material, and then if it bounces off, that's reflection. Now the other thing that light can do is get absorbed. Absorbed is when the material doesn't reflect any and doesn't transmit any through it, if it's completely absorbing. And so that would be a situation like this. Now the reality in the real world is we have oftentimes a little bit of all of the above happening. For example, when we look at this white snow up here, this is reflecting pretty well. So there's light that's coming from the sun, or maybe it's being reflected off the clouds, or maybe it's getting through the clouds, and when it hits that snow, it gets reflected."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "And so that would be a situation like this. Now the reality in the real world is we have oftentimes a little bit of all of the above happening. For example, when we look at this white snow up here, this is reflecting pretty well. So there's light that's coming from the sun, or maybe it's being reflected off the clouds, or maybe it's getting through the clouds, and when it hits that snow, it gets reflected. Now the reason why we don't see a reflection the way that we see in this lake right over here is the snow reflects it in all different directions. The fact that it's that bright color, in fact, you might need sunglasses to look at the snow just as much as you need to look at the sky, is that it's reflecting most of the light. But if you go down here where we see the trees, the same light from the sky is hitting it, but not as much light is coming back to our eye."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "So there's light that's coming from the sun, or maybe it's being reflected off the clouds, or maybe it's getting through the clouds, and when it hits that snow, it gets reflected. Now the reason why we don't see a reflection the way that we see in this lake right over here is the snow reflects it in all different directions. The fact that it's that bright color, in fact, you might need sunglasses to look at the snow just as much as you need to look at the sky, is that it's reflecting most of the light. But if you go down here where we see the trees, the same light from the sky is hitting it, but not as much light is coming back to our eye. And that's because this part of the mountain, it might be trees, it might be rock, it might be dirt, is absorbing more of the light. But it's still reflecting some. We can still see it a little bit."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "But if you go down here where we see the trees, the same light from the sky is hitting it, but not as much light is coming back to our eye. And that's because this part of the mountain, it might be trees, it might be rock, it might be dirt, is absorbing more of the light. But it's still reflecting some. We can still see it a little bit. So this might be where most of it is getting absorbed, but a little bit of the light gets reflected, which we can see right over there. And if we think about the water right over here, some of the light is probably making it through and probably refracting as it does so. Some of it is getting absorbed as it makes its way through the water."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "We can still see it a little bit. So this might be where most of it is getting absorbed, but a little bit of the light gets reflected, which we can see right over there. And if we think about the water right over here, some of the light is probably making it through and probably refracting as it does so. Some of it is getting absorbed as it makes its way through the water. If this was a really deep lake, when you get to the bottom, it could be very, very dark. And then as we talked about, it looks like a good bit is reflecting. When we look at the image that we see in the lake, it looks almost as bright as the real thing."}, {"video_title": "Absorption and reflection   Waves   Middle school physics   Khan Academy.mp3", "Sentence": "Some of it is getting absorbed as it makes its way through the water. If this was a really deep lake, when you get to the bottom, it could be very, very dark. And then as we talked about, it looks like a good bit is reflecting. When we look at the image that we see in the lake, it looks almost as bright as the real thing. So as I said, this is happening all around you. In fact, when people make fancy computer graphics, they actually try to do exactly what the light would do in the real world to make an image for your eyes that look like the real world. And they're thinking exactly about this, how much gets through a substance, how much gets reflected, and how much gets absorbed."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "When you hear the word gravity, you probably just think of things falling like an apple from a tree. But did you know it's also the reason why your lamp is staying on the floor? That's because gravity is so much more than things falling down. Gravitational forces are these invisible forces that pull objects together. So gravitational force is actually attracting the lamp to the floor. And these forces exist between all objects with mass. So let's write these key points out about gravitational forces, which I'm going to use GF to represent."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Gravitational forces are these invisible forces that pull objects together. So gravitational force is actually attracting the lamp to the floor. And these forces exist between all objects with mass. So let's write these key points out about gravitational forces, which I'm going to use GF to represent. We said they are attractive forces and that they exist between all objects with mass. Objects with mass. To explain this, we first need to remember a couple of things."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "So let's write these key points out about gravitational forces, which I'm going to use GF to represent. We said they are attractive forces and that they exist between all objects with mass. Objects with mass. To explain this, we first need to remember a couple of things. Mass is how much matter objects have, and matter is the stuff an object is made of. Any object with mass generates a gravitational pull, so there is a gravitational force of attraction between every object. The amount of gravitational force between two objects will depend on two things."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "To explain this, we first need to remember a couple of things. Mass is how much matter objects have, and matter is the stuff an object is made of. Any object with mass generates a gravitational pull, so there is a gravitational force of attraction between every object. The amount of gravitational force between two objects will depend on two things. The masses of the two objects and the distance between them. The mass of each object is proportional to the gravitational force. This means that the more mass an object has, the stronger its gravitational force."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "The amount of gravitational force between two objects will depend on two things. The masses of the two objects and the distance between them. The mass of each object is proportional to the gravitational force. This means that the more mass an object has, the stronger its gravitational force. And now we can understand why gravity makes things fall. The Earth is massive, literally. It's almost six septillion kilograms."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "This means that the more mass an object has, the stronger its gravitational force. And now we can understand why gravity makes things fall. The Earth is massive, literally. It's almost six septillion kilograms. That's a six with 24 zeros after it. So it generates a huge attractive force. For comparison, my lamp is only one kilogram, which is why if I jump, I fall towards the Earth and not towards my lamp."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "It's almost six septillion kilograms. That's a six with 24 zeros after it. So it generates a huge attractive force. For comparison, my lamp is only one kilogram, which is why if I jump, I fall towards the Earth and not towards my lamp. But we said the mass of the object is just one factor affecting the strength of its gravitational force. The other is the distance between objects. The more distance between the objects, the weaker the gravitational pull between them."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "For comparison, my lamp is only one kilogram, which is why if I jump, I fall towards the Earth and not towards my lamp. But we said the mass of the object is just one factor affecting the strength of its gravitational force. The other is the distance between objects. The more distance between the objects, the weaker the gravitational pull between them. For small objects without much mass, it doesn't take much distance for their gravitational forces between each other to be so weak that we don't notice them. For something like the Earth, you have to go really far away to not be affected by its gravitational force of attraction. I mean, look at the Moon."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "The more distance between the objects, the weaker the gravitational pull between them. For small objects without much mass, it doesn't take much distance for their gravitational forces between each other to be so weak that we don't notice them. For something like the Earth, you have to go really far away to not be affected by its gravitational force of attraction. I mean, look at the Moon. It's almost 240,000 miles away. That's almost 400,000 kilometers. And it still feels effects from Earth's gravity."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "I mean, look at the Moon. It's almost 240,000 miles away. That's almost 400,000 kilometers. And it still feels effects from Earth's gravity. That's why it's orbiting us. But since the Moon is also a pretty massive object, we do experience the effects of its gravitational pull on the Earth. This is why we have tides."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "And it still feels effects from Earth's gravity. That's why it's orbiting us. But since the Moon is also a pretty massive object, we do experience the effects of its gravitational pull on the Earth. This is why we have tides. The Moon's gravitational force will pull on Earth's water, which results in us having high and low tides. Now you might be wondering if gravity can affect the Moon's gravitational pull. Or cause tides."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "This is why we have tides. The Moon's gravitational force will pull on Earth's water, which results in us having high and low tides. Now you might be wondering if gravity can affect the Moon's gravitational pull. Or cause tides. How can we even move around? Why don't we just face plant it on the ground because Earth's gravity is pulling us towards it? It turns out that actually gravity is a pretty weak force."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "Or cause tides. How can we even move around? Why don't we just face plant it on the ground because Earth's gravity is pulling us towards it? It turns out that actually gravity is a pretty weak force. We only even notice its effects when an object is massive like planets or stars. And the gravitational force on you is way weaker than most forces you exert every day. In fact, every time you pick up a glass of water, you feel it."}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "It turns out that actually gravity is a pretty weak force. We only even notice its effects when an object is massive like planets or stars. And the gravitational force on you is way weaker than most forces you exert every day. In fact, every time you pick up a glass of water, you feel it. In fact, every time you pick up a glass of water, you're overpowering the entire mass of Earth. How cool is that?"}, {"video_title": "Gravitational forces   Forces at a distance   Middle school physics   Khan Academy.mp3", "Sentence": "In fact, every time you pick up a glass of water, you feel it. In fact, every time you pick up a glass of water, you're overpowering the entire mass of Earth. How cool is that?"}]