question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>In robotics, there exists drive-trains that can move omni-directionally (that is in any direction). These come in many shapes and sizes, but most come in a three wheel or four wheel configuration, to keep things simple lets assume three wheels. </p>
<p><img src="http://i.stack.imgur.com/Pt7ZH.png" alt="Three wheel ... | 5,739 |
<p>I wanted to know what the observable universe is so I was thinking and I thought, it must be age of the universe times 2.</p>
<p>Well I was wrong. I found on one website that it is 46B LY across in each direction. How does this make sense?</p>
<p>I get how the universe has expanded since then, but we should only b... | 10 |
<p>In Polchinski's book "String theory", he is saying "Unlike the case of D-branes, there are no string modes tied to the orientifold plane to represent fluctuations in its shape. Our heuristic argument that a gravitational wave forces a D-brane to oscillate does not apply to the orientifold plane. Essentially, the ide... | 5,740 |
<p>I know and understand why equation below holds. But i am new to operator thing in QM and would need some explaination on this. </p>
<p>$$\langle x \rangle = \int\limits_{-\infty}^\infty |\Psi|^2 x \, \mathrm d x $$</p>
<p><strong>Q1:</strong> Is it ok if i interpret the equation above like quoted:</p>
<blockquote... | 5,741 |
<p>How does a <a href="http://en.wikipedia.org/wiki/Fresnel_rhomb" rel="nofollow">Fresnel rhomb</a> work (half and quarter wave plate)?</p>
<p>I am aware of <a href="http://en.wikipedia.org/wiki/Birefringence" rel="nofollow">birefringence</a>, which creates a phase shift of $\Delta\phi=\dfrac{2\pi\Delta nL}{\lambda_0}... | 5,742 |
<p>I don't believe the following is an accurate description of the physical but a homework problem to help understanding.</p>
<blockquote>
<p>A beam of electron of energy 0.025 eV moving along x-direction, passes
through a slit of variable width w placed along y-axis. Estimate the
value of the width of the slit ... | 5,743 |
<p>Every kid who first looks into a telescope is shocked to see that everything's black and white. The pretty colors, like those in this picture of the <a href="http://apod.nasa.gov/apod/ap110612.html">Sleeping Beauty Galaxy (M64)</a>, are missing:</p>
<p><a href="http://apod.nasa.gov/apod/image/1106/m64_hst_897.jpg"... | 5,744 |
<p>The lenses of my telescope and binoculars are dusty. What is the best way to clean them without damaging the optic coating?</p> | 5,745 |
<p>The Schwarzschild metric reduces to the Minkowski metric in the limit of vanishing $M$, but the Hawking temperature which is proportional to $1/M$ diverges in the same limit. This would imply that flat spacetime has infinite rather than zero temperature. What am I missing?</p>
<p>EDIT: This question is back up on t... | 5,746 |
<p>Imagine we have cup A with 50 g of water and cup B (smaller in width than A) with 100 g of water. Now put cup B into cup A. If the width of both cups are of comparable size then the cup with 100 g of water floats. It does not touch the bottom of cup B. </p>
<p>Now think about <a href="http://en.wikip... | 151 |
<p>I have preformed the muon lifetime experiment at my uni's lab, and got the data. It's text file with 8190 numbers. My TDC unit was set so that the time gates were at 10 $\mu s$, and it has 8192 channels (first and last contained some kind of noise so they were not included in data analysis).</p>
<p>Now, I made a pr... | 5,747 |
<p>I'm designing a copter and trying to decide if the propellers should be ducted or open axial flow. I've read some theory on ducted and open air flow but I can't find any where that compares the two. I would prefer to use ducted over open for safety (ie put a safety guard round the propellers so they can't be run int... | 5,748 |
<p>I am trying to analyse the problem of sticking an aluminum piece to a stainless steel piece only with pressure. The holding capacity of the system is determined by the static friction between the two pieces.</p>
<p>To improve improve the holding capacity I would like to introduce a polymer layer between the two pie... | 5,749 |
<p>Linked to this question: <a href="http://physics.stackexchange.com/q/64780/">Comparing Static Frictions</a></p>
<p>Suppose there is a cuboidal vertical tunnel, and a cubical block in it such that all surface of the block are in contact with the four walls of the cuboidal tunnel respectively. All walls of the tunnel... | 5,750 |
<p>1) The bent rod $ABCD$ rotates about the line $AD$ whit a constant angular velocity of $90 rad / s$. Determine the velocity and acceleration of the vertex $B$ when the rod is in the position shown in Figure.</p>
<p>2) Determine the velocity and acceleration of the vertex $B$ assuming now that the angular velocity i... | 5,751 |
<p>Suppose you have a particle of mass $m$ fixed to a spring of mass $m_0$ that, in turn, is fixed to some wall. I'm trying to calculate the effective mass $m'$ that appears in the law of motion of the particle (suppose the system is isolated):$$m'\ddot x=-k(x-x_0).$$
I've read somewhere that this should be $m'=m+m_0/3... | 5,752 |
<p>My question may not be suitable here, because it's more of astrobiology. Life as we know it is carbon based. </p>
<ol>
<li>Is life based on silicon possible?</li>
<li>What would the conditions for habitability for silicon based life be?</li>
<li>Can both carbon- and silicon-based life co-exist?</li>
<li>Is any sili... | 5,753 |
<p>I've searched various publication archives but I'm a signal processing engineer and not an applied physicist, so I might not be using the right terms. While I can find various average values for the flux of the sky during the day (no clouds) (in both psychometric and radiometric units), what I'm really interested in... | 5,754 |
<p>I have always fancied building my own reflector telescope. I am wondering - For a certain budget, can you get better results with a home made rig? Or is this a folly, and really it would be better to buy a ready made unit?</p> | 5,755 |
<p>I have read that noise is a result of there being "no such thing as a perfect one-way valve". That energy transfer is never perfectly one-way; there will always (at some level) a finite flow of energy from output to input.</p>
<p>I understand this much, but what is the fundamental reason for this? Does it have its ... | 5,756 |
<p>Why, when one releases 2 balls in Newton's cradle, two balls on the opposite side bounce out at approximately the same speed as the 1st pair, rather than one ball at higher speed, or 3 balls at lower speed?</p> | 5,757 |
<p>This is a simple question but it seems to me that both the explanations are acceptable.</p>
<p>Let say the least distance of least distinguished vision is 25 cm.</p>
<p>An object is placed 12.5 cm in front of a plain mirror. Where should my eyes be placed such that I can see the image clearly?</p>
<p>Actually, th... | 5,758 |
<p>This question was triggered by a comment of Peter Shor's (he is a skeptic, it seems.) I thought that the holographic principle and AdS/CFT dealt with that, and was enough for Hawking to give John Preskill a baseball encyclopedia; but Kip Thorne is holding out, as are others. Why?</p> | 207 |
<p>Does the expectation value of an observable must be equal to an eigenvalue of the corresponding operator?
I already know that 0 is not an eigenvalue, but is there any other examples?</p> | 5,759 |
<p>This question is in reference to <a href="http://arxiv.org/abs/arXiv%3ahep-th/0112258" rel="nofollow">this</a> very famous paper of Witten. </p>
<ul>
<li><p>In general through the whole paper why is the author able to just focus on the scalar field propagating in the bulk and not need to take into account all the o... | 5,760 |
<p>I was recently driving behind a car that had a bicycle mounted on a carrier over the rear bumper.</p>
<p>The bicycle wheels were not bound so they were rotating in the slip-stream of the car.
I wonder, the fact that the wheels are turning; does this theoretically increase or decrease the drag on the car?</p> | 5,761 |
<p>I am trying to explain to myself the motivation behind studying the asymptotic structures at null, time-like and space-like infinities (For the purposes of this post, I will stick to four dimensional Minkowski spacetime). I think I have the motivation behind the first two of them down, but not the third.</p>
<ol>
<... | 5,762 |
<p><strong>Problem.</strong> I know that the two wave functions $\Psi_1$ and $\Psi_2$ are all normalized and orthogonal. I now want to prove that this implies that $\Psi_3=\Psi_1+\Psi_2$ is orthogonal to $\Psi_4=\Psi_1-\Psi_2$. </p>
<p><strong>My naive solution.</strong> From the premises, we know that
$$\int_{-\inft... | 5,763 |
<p>Recently I learned functional integral's formalism in QFT. I have realized that I don't understand why exactly do we introduce it. We have the expression for $S$-matrix, then we may rewrite it in through functional integral. </p>
<p>But why do we use functional integration? I read that it helps to "conserve" gauge ... | 5,764 |
<p>This is a thing Iïve seen on many papers dealing with Warped Extra Dimensions, specifically on slices of AdS5. But the one where it appears more clearly is a lecture by Tony Gherghetta:</p>
<p><a href="http://arxiv.org/abs/hep-ph/0601213" rel="nofollow">http://arxiv.org/abs/hep-ph/0601213</a></p>
<p>Essentially wh... | 5,765 |
<p>I am working through Yale's Physics 201, and on the first problem set there is a question that is puzzling me. From my reasoning, I think the answer they provided is incorrect hence why I am here. The question is as follows: </p>
<blockquote>
<p>Two sphere of mass m and negligible size are connected to two identi... | 5,766 |
<p>I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the Ampere that is the base unit and not the Coulomb. Why is this? It seems to me that charge can exist without current, but current cannot exist without charge. So the logical choice for a base un... | 869 |
<p>My question is: Can an electron volt be considered an alternate unit for electric potential? </p> | 5,767 |
<p>What i always thought that air conditioners blow the cool air without knowing that they actually take the warm air from indoor and from outdoor. But whats the point of taking air from indoor and outdoor, Shouldn't they just be blowing the cool air inside and out air outside? How does it really make the difference?</... | 5,768 |
<p>It is posible to define a good variational principle to describe Fluid Mechanics? if so, wath is the correct tratement of the issue. I guess something like:</p>
<p>$I=\int d^4x (\frac{1}{2}\rho v^2-P-\rho g x)$</p> | 5,769 |
<p>I am trying to do an aerodynamic drag equation on a descending parachute (the round variety) and have no idea what the reference area on one would be. I know for a sphere, you can use radius*radius*PI to get the reference area. Is that the same for a parachute?</p> | 5,770 |
<p>Hubble's law says that the universe is expanding.How come the milky way and the andromeda are on a collision course?How will they end up colliding with each other?</p> | 5,771 |
<p>When Yang-Mills field theory was introduced, a problem is that the gauge invariance can not allow mass for the gauge field. Later people invented spontaneous symmetry breaking and Higgs mechanism to give the gauge field mass. The Higgs particle is almost confirmed at LHC.</p>
<p>My question is, since there is symme... | 5,772 |
<p>Is Pluto made out of ice--as in frozen water?
If so, wouldn't that provide evidence for alien life in the cosmos?</p> | 5,773 |
<p>See <a href="http://peswiki.com/index.php/Directory:Milkovic_Two-Stage_Mechanical_Oscillator" rel="nofollow">http://peswiki.com/index.php/Directory:Milkovic_Two-Stage_Mechanical_Oscillator</a> </p>
<p>The Two-Stage Mechanical Oscillator Pendulum-Lever System is very simple, yet very puzzling because it appears that... | 5,774 |
<p>Liquid electrolytes ionize and hence a current can pass through them. So if a gas can ionize, can it conduct electricity too? If so, what are a few such gases?</p> | 5,775 |
<p>I have just started studying Microphones and Loudspeakers. I need a good text to refer which can explain their mechanical analogies with simplicity and basics too. </p> | 5,776 |
<p>A stack of N D-branes can have open strings ending on them. There is a U(N) brane gauge field, and r adjoint Higgs fields, with r equal to the number of transverse spatial dimensions. The eigenvalues of the Higgs fields gives the (noncommutative) brane displacements.</p>
<p>Surprisingly, this is equivalent to an ex... | 5,777 |
<p>I've read that light travels in a straight line and has a wavelength of 400nm to 700nm. But I don't understand why does it have a wavelength and what creates its wavelength? I agree with the concept of sound which also has wavelength, thus called sound waves which are created by the vibrational movement in air. I'm ... | 5,778 |
<p>Why is a white cloud is formed in the hot water from the pot when salt (NaCl) is added? </p> | 5,779 |
<p>A non-spinning, equally massive black hole and white hole experience a direct collision. </p>
<p>What shall happen? What shall be the result of such a collision?</p> | 5,780 |
<p>I'm reading a book on electromagnetism and I am a bit confused about some things in Maxwells equations. This is what I don't like about many physics books: they are very wordy, but at the end you don't know what is an experimental fact, what is a "theorem", what is an assumption and so on,...</p>
<p>Anyway the ques... | 5,781 |
<p>Theoretically if I were able to build some sort of device that let me sit 1 foot away from the surface of the Sun (or any star for that matter) without being vaporized, would a star produce any sort of audible sound at that distance? From my understanding, sound waves need a medium to propagate through (air, water... | 882 |
<p>I have a question regarding, as the title says, this equation: $\nabla \times \textbf{E}=-\frac{\partial \textbf{B}}{\partial{t}}$</p>
<p>So, the above equation says that the curl of an electric field is proportional to the rate of change of a magnetic field. However, since electric fields have a potential associat... | 5,782 |
<p>If you have a uranium atom of radius $10^{-14}m$ that undergoes fission into two fragments each with 46 protons and radius $8\times10^{-15}m$. There are two ways you can calculate the electrostatic energy realised (according to my textbook):</p>
<ol>
<li>Find the electric potential energy of the two fragments as if... | 5,783 |
<p>I'm currently studying Goldstein's Classical Mechanics book and I can't get my head around his reasoning in section 2.4. (Extending Hamilton's principle to systems with constraints).
I'd like to understand the example he gives. Here it comes:</p>
<hr>
<p>Consider a smooth solid hemisphere of radius $a$ placed wit... | 5,784 |
<p>I have two laser beams with same polarization running parallel to each other. Will they interfere? If yes, then what are the conditions (perpendicular distance etc) and how can I observe the interference pattern? </p> | 5,785 |
<p><img src="http://i.stack.imgur.com/zH5ft.png" alt="I don't understand this proof"></p>
<p>Could anyone tell me $\hat {\bf n}\cdot{\bf \sigma}$ is defined in such way? In the book they have not defined what is $n_z,n_x,n_y$.</p>
<p>It is from <em>Quantum Computing: From Linear Algebra to Physical Realization</e... | 5,786 |
<p>The question for the full derivation of Probability Conservation -> Probability Current was already asked here:
<a href="http://physics.stackexchange.com/questions/15790/probability-current">Probability current</a>. I apologize for not retyping it out, but it's already beautifully done in latex which I have yet to... | 5,787 |
<p>I may not understand osmosis very well. Let us suppose two compartments filled with water, separated by a semi-permeable membrane. At equilibrium, both levels are equals. Let us introduce now a given volume of solute in one of the compartments (say right). At first, the level of the right compartment will increase, ... | 5,788 |
<p>I am in a low level physics class that is taught in high school. We were given a couple formulas as seen on the board, but I don't know what the variables stand for (their names), and what units correspond to each variable.</p>
<p>I have searched a few formulas on Google word for word (4), but nothing positive came... | 5,789 |
<p>Let's say I shine a laser from a stationary medium into a moving medium (suppose the water is moving very quickly) perpendicular to the interface and back to a stationary medium like this:</p>
<p><img src="http://i.stack.imgur.com/SiL6P.png" alt="Scenarios"></p>
<p>(Note: left and right sides of the image are stat... | 5,790 |
<p>I just found out that electrolytic capacitors hardly leak whereas non-electrolytic capacitors leak and at times, leak a lot. Questions: </p>
<p>(a) what causes capacitors to leak? </p>
<p>(b) why do electrolytic capacitors hardly leak when wired correctly whereas if they are reversed, they leak a fair amount?</p>
... | 5,791 |
<p>Some time ago I came across this simple task. Instead of describing it a lot I will just post a picture and try to explain like that.</p>
<p><img src="http://i.stack.imgur.com/V9SuT.jpg" alt="enter image description here"></p>
<p>So the man and the weight on the other side of the rope have the same mass, everythin... | 799 |
<p>I would like to minimize a functional of the type:
$$L[\gamma]=\int_a^b F(T(\gamma(t))dt$$
on the space of paths $\gamma$, where $T=T(\gamma,t)$. Now, usually I would simply apply Euler-Lagrange's equations, but in this case $T$ is defined implicitly by an equation of type:
$$f(t,\gamma,\dot\gamma,\ddot\gamma,T)=0$$... | 5,792 |
<p>Why after the expansion, only 0-mode of bc-ghost contributes to the 4-points ghost function on a torus worldsheet?</p>
<p>$$<c(z_1)b(z_2)\tilde{c}(\bar{z}_3)\tilde{b}(\bar{z}_4)>_{T^2} ~\longrightarrow~ <c_0 b_0 \tilde{c}_0 \tilde{b}_0>$$</p> | 5,793 |
<p>I have always been excited by examples in which a few simple assumptions and first principles are used to characterize a system. For example, I did an exercise in which Crawford estimates a lake to be rectangular and then predicts the frequency of its waves through the harmonic motion of its center of mass.</p>
<p... | 5,794 |
<p>I wanted to calculate the moment of inertia of a ring about an axis at 45° to its normal outside the plane of the ring . </p>
<p>How do i calculate without using integration? I was thinking about using perpendicular axis theorom , but I need 2 axis in the plane of ring which I was unable to find . Please help</p> | 5,795 |
<p>When treating QFT we want our theory to be invariant under different symmetry groups, for example, the Standard Model is a non-abelian gauge theory with the symmetry group $U(1)×SU(2)×SU(3)$. Moreover, it is invariant under Lorentz transformations which form the Lorentz group $O(1,3)$ with the proper transformations... | 5,796 |
<p>I am preparing myself for an exam and I got stuck with the following problem.</p>
<p>If I wanted to calculate the vector potential $A$ on a sphere (not off or in), where some points are removed, how would I have to tackle this problem effectively?</p>
<p>If I remove a point on the $2$-plane the standard solution i... | 5,797 |
<p>Is $E=mc^2$ not just $E=m$. What does the speed of light have to do with this other than to give it a really big number so it looks cool? What spectrum of light is used? How can we test the speed of light with out a stationary point to test it from?</p> | 5,798 |
<p>It's pretty rainy in Boston now, and there are streams of water by the sides of the street.</p>
<p>Why do they have a 'woven' appearance? What's the formal name for this kind of fluid flow? With lots of diagonals criss-crossing each other. And is there a good way to simulate it?</p> | 5,799 |
<p>I've started today the book of Landau and Lifshitz Vol.2: <em>The Classical Theory of Fields</em> $\S 2$. They start from the invariance of the speed of light, express it as the fact that $$c^2(\Delta t)^2-(\Delta x)^2-(\Delta y)^2-(\Delta z)^2=0$$ is preserved when we change inertial frame, so they consider $$ds^2=... | 5,800 |
<p>Suppose we have several interacting particles in pure state $\left|\psi\right>$. For each of particles we can extract density matrices via</p>
<p>$$\rho_i(x_i,x_i^\prime)=\int \left<X_i,x_i\middle|\psi\right>\left<\psi\middle|X_i,x_i^\prime\right>dX_i,$$</p>
<p>where $x_i$ is coordinates of particle... | 5,801 |
<p><img src="http://i.stack.imgur.com/UhwNG.jpg" alt="enter image description here"></p>
<p>Translation first!<br>
The figure shows an electric circuit.<br>
a) Use Kirchhoffs 2.law on the part-circuit abed and show that $I_3$ = 0,50A.<br>
b) Explain that $I_2 + I_5 = 2,0A$. Use Kirchhoffs 2.law on the part-curcuit bef... | 5,802 |
<p>If there where no friction at all, would a spinning wheel held up by one end of the axis spin precess forever without falling down? </p>
<p><img src="http://i.stack.imgur.com/S3f2n.gif" alt="Spinning wheel"></p>
<p>I just asked another question about the same problem:</p>
<p><a href="http://physics.stackexchange.... | 5,803 |
<p>Consider a spinning wheel, which is held up by one end of it's axis like this: <img src="http://i.stack.imgur.com/bGYsu.gif" alt="Spinning wheel from hyperphysics"></p>
<p>To explain why the change of angular momentum is directed as shown in the figure above, one usually says that there is an applied torque $\vec{\... | 5,804 |
<p>Trying to teach myself general relativity and have just hit yet another confusion. I'm reading that in curved spacetime the energy-momentum tensor has zero divergence, ie</p>
<p>$$\nabla_{\mu}T^{\mu\nu}=0.$$</p>
<p>But that this doesn't imply the total conservation of energy and momentum as there is an additional ... | 5,805 |
<p>Why is physical space equivalent to $\mathbb{R}^3$, as opposed to e.g. $\mathbb{Q}^3$?</p>
<p>I am trying to understand what would be the logical reasons behind our assumption that our physical space is equivalent to $\mathbb{R}^3$ or 'physical straight line' is equivalent to $\mathbb{R}$ . </p>
<p>The set of rea... | 5,806 |
<p>The energy eigenstates of the infinite square well problem look like the Fourier basis of L2 on the interval of the well. So then we should be able to for example make square waves that are an infinite linear combination of those energy eigenstates. But since linear superpositions of solutions are solutions, does th... | 5,807 |
<p>Klein-Gordon equation in quantum field theory is known to suffer from the possibility of negative probability. So, the question is, despite this, Klein-Gordon describes spin-zero field. So, how can negative probability and scalar field co-exist?</p> | 5,808 |
<p>A ball is shot directly upward, and then it comes back to the place where it was shot. Suppose we have air resistance. Suppose $t_1$ is the time period from the moment that the ball was shot to the moment that it reached its highest altitude, and $t_2$ is the time period from the moment it reached its highest altitu... | 5,809 |
<p>Just as a thought experiment. </p>
<p>One factor in the economies of space exploration is that of fuel. This may be split as </p>
<ul>
<li>MSL - Earth Orbit </li>
<li>Earth Orbit to Inner Solar System </li>
<li>Beyond.</li>
</ul>
<p>In each of these cases, the rocket demands an oxidizer to be loaded at the point ... | 5,810 |
<blockquote>
<p>Let us divide the time $T$ into $N$ segments each lasting $δt = T/N$. Then
we write $\langle q_F | e^{−iHT} |q_I \rangle = \langle q_F | e^{−iHδt} e^{−iHδt} . . . e^{−iHδt} |q_I \rangle $
Our states are normalized by $\langle q' | q \rangle = δ(q' − q)$ with $δ$ the Dirac
delta function. ("Dira... | 5,811 |
<p>In 1 dimension what is the solution of the Schrödinger equation with potential</p>
<p>$$ V(x) = V_r + i V_i $$</p>
<p>Potentials are constant.</p> | 5,812 |
<p><strong>Question.</strong> <em>Are there any references or techniques which can be applied to obtain energy gaps for ferromagnetic XXX spin-1/2 Hamitlonians, on general interaction graphs, or <a href="http://en.wikipedia.org/wiki/Tree_%28graph_theory%29" rel="nofollow">tree-graphs</a>?</em> </p>
<p>I'm interested i... | 5,813 |
<p>There's something known as a "resonating valence bond" (RVB) state, which plays a role in at least some attempts to understand physics of high-$T_c$ superconductors. This, roughly, involves a state that's in a superposition (hence the "resonating" part of the name, if I understand correctly) of different ways to pai... | 5,814 |
<p>I've seen a documentary, whose name I don't remember but I'm curious because it suggests that subatomic particles are able to "foresee the future".</p>
<p>I'll try to describe it here:</p>
<p>Some particles are shot through two slits in a plate, if the particles are not interfered by trying to measure its trajecto... | 5,815 |
<p>This is a follow-up question to <a href="http://physics.stackexchange.com/questions/44120/what-are-wightman-fields-functions/44142#comment91472_44142">What are Wightman fields/functions</a></p>
<p>Ok, so based on my reading, the field operators of a theory are understood to be operator-valued distributions, that is... | 5,816 |
<p>I run some battery powered test equipment in close proximity (~10cm) to a conductor carrying 25kV (50Hz AC). I find the battery life is much shorter than expected, having checked the current consumption of the equipment. Could the electric field adversely affect battery operation? The battery is a primary lithium th... | 5,817 |
<p>Some time ago I posted a <a href="http://physics.stackexchange.com/questions/19356/have-experiments-ever-suggested-two-different-values-to-the-same-divergent-serie">question</a> here on this forum. I would like to ask some questions regarding the way the energy per unit area between metallic plates is calculated. Th... | 5,818 |
<p>As I'm very well convinced for now, the better understanding of energy we have today in Physics is that of a conserved quantity, i.e, a numerical valued associated with a system that remains the same whatever process the system undergoes.</p>
<p>This is all fine, but then we have areas in Physics where we talk of f... | 5,819 |
<p>I'm working on a problem where a particle of mass $m$ is confined to the surface of an inverted half cone (and is circling downwards due to gravity), with the cone's half angle $\alpha$. I chose to use cylindrical coordinates $(z,\phi,\rho)$ and I used the Lagrangian to solve this problem.</p>
<p>After going throug... | 5,820 |
<p>In first order of perturbation theory the S-matrix amplitude for electron scattering in the Coulomb field will be (up to normalization factors)
$$
S_{fi} = \frac{iZ q^2}{\sqrt{2E_{f}2E_{i}}}\bar {u}(p_{f}, s_{f})\gamma_{0}u (p_{i}, s_{i}) \int \frac{d^{4}xe^{i(p_{f} - p_{i})x}}{|\mathbf x|},
$$
where $f, i$-indices ... | 5,821 |
<p>We say that sound waves need a medium to propagate and we know that light doesn't need such a thing. But is that really how that works? There's no such thing as "nothing" according to Quantum Mechanics, so I'm wondering if light can travel through "perfect vacuum"? A vacuum that there's no field, no lowest energy th... | 623 |
<p>I have a very simple question. A motorboat directed upstream is seen to be at rest from the bank of a river. Is the engine doing any work? Is it right to say that since it is not causing any displacement, so it is not doing any work.</p> | 5,822 |
<p>I hope the experts of the field forgive me for this n00b questions, but I am just trying to understand physics. Assume the following function:</p>
<p>$$\phi(t)=\omega t+\cos(\omega t)$$</p>
<p>The above function has the property that:</p>
<p>$$\phi(t+T)-\phi(t)=\omega T=const.$$
Where $T=\frac{2\pi}{\omega}$.</p>... | 5,823 |
<p>I can't think of a good title for my post, sorry about that. </p>
<p>I've got a lunch box (called a <a href="https://www.google.com/search?q=bento+box&hl=en&tbm=shop&prmd=imvnse&source=lnms&sa=X&ei=LgB3UPerHOrq0gHFpIDIAg&ved=0CA4Q_AUoBQ&biw=1343&bih=782" rel="nofollow">bento box<... | 5,824 |
<p>I remember reading a brief note in Scientific American years ago about a mathematician/physicist who had published a paper that formally stated that no entity could both participate in a given system and simultaneously know all information about that system. His conclusions were not particularly ground breaking for... | 5,825 |
<p>I have come across many hand dryers that attempt to dry your hands really fast after you wash them. Here are two of them:</p>
<p>XLERATOR
<a href="http://www.exceldryer.com/" rel="nofollow">http://www.exceldryer.com/</a></p>
<p>Dyson Airblade
<a href="http://www.dysonairblade.com/homepage.asp" rel="nofollow">http:... | 5,826 |
<p>I wonder, why the receiver of this solar tower is not black? Wouldn't a black receiver be more efficient? If not, why is it not white then, but is an intermediate color?</p>
<p><img src="http://i.stack.imgur.com/qrWIF.jpg" alt="enter image description here"></p>
<p>It seems that the receiver of other solar towers,... | 5,827 |
<p>Why does the <a href="http://en.wikipedia.org/wiki/Principle_of_locality" rel="nofollow">principle of locality</a> have so such great importance in physics that theory should be consistent with it?</p> | 5,828 |
<p>The book <em>Commonly Asked Questions in Thermodynamics</em> states:</p>
<blockquote>
<p>When we refer to the passage of the system through a sequence of internal equilibrium states without the establishment of equilibrium with the surroundings this is referred to as a reversible change. An example that combines ... | 5,829 |
<blockquote>
<p>A yacht on a lake drops its anchor overboard. What happens to the water level in the lake? </p>
<ol>
<li>It rises very slightly.</li>
<li>It falls very slightly.</li>
<li>It stays exactly the same.</li>
<li>It's impossible to say.</li>
</ol>
</blockquote>
<p>My understanding is that du... | 5,830 |
<blockquote>
<p>The generator in a nuclear reactor delivers 580 MW electric power.</p>
<p><strong>a)</strong> How much TWh electrical energy is delivered yearly if the reactor runs only 70 % of the time?</p>
<p><strong>b)</strong> What flow of coolant (water) is required if the efficiency of the steam turbi... | 5,831 |
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