question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>An <a href="http://en.wikipedia.org/wiki/Alcubierre_drive" rel="nofollow">Alcubierre Drive</a> is a hypothetical device that can move a place someplace else faster than the speed of light without violating known laws of physics.
<a href="http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015936_2011016932.pd... | 5,285 |
<p>In classical computer simulations such as molecular dynamics (MD) simulations, one integrates Newton's equations of motion to determine particle trajectories. If we think of Newton's Second Law as simply $$\textbf{f}_i = \frac{d \textbf{p}_i}{dt}$$ where $\textbf{f}_i$ is the net force acting on a particle $i$ and ... | 5,286 |
<p>Weinberg claims that it is obvious that the $\sigma = 0$ component of $u^\mu$ at zero spatial momentum points in the 3-direction. This is supposed to follow from (5.3.6). Unfortunately I am not seeing it. I thought that the $J=1, m = 0$ component is conventionally aligned in the 2-direction?</p>
<p>Any help would ... | 5,287 |
<p>I want to know how the flow of electrons will change when I change the tempertature from 100 F to 250 F in a silicon semi-conductor (ex: computer mouse)</p>
<p>How can I find this out?</p> | 5,288 |
<p>I have again an old question from a comprehensive exam I took a couple of months ago. Lucky for me one could pick 5 out of 8 questions, because on some of the problems I didn't even know how to start. Now that classes are over I've now the time to revisit those problems I was dumbfounded by, such as this one: (Abrig... | 5,289 |
<p>In my physics book "University Physics", there is a chapter on relating linear and angular kinematics.</p>
<p>I understand the parts where it shows $v = r\omega$ and $a_{\text{tan}} = r\alpha$.</p>
<p>However in the part where they show $a_{\text{rad}} = r\omega^2$, they use the formula $a_{\text{rad}} = \frac{v^2... | 5,290 |
<p>I'm working on an inertial measurement unit with some MEM'S component. I want to retrieve data on a micro-controller. I have 3-axis accelerometer sensor. however I want retrieve data in $mg$ ( $g=9.81$ $m/s^2$ so $mg = 0.001g = 0.00981m/s^2$) .</p>
<p>I just would like to calculate velocity in $meters/s$ .</p>
<p>... | 5,291 |
<p>I am reading a book on fundamental physics for mechanics. There is a truss shown in the figure. A train of mass $M=56$ ton is resting at the middle point of AC, ignore all the mass of truss and rails. All triangles are equilateral. I am trying to find the force exerted on strut AB. </p>
<p><img src="http://i.stack.... | 5,292 |
<p>In <em>Philosophy of Language</em> by William G. Lycan, there are the lines:</p>
<blockquote>
<p><em>Even apparent truths of logic, such as truths of the form "Either P or not P", might be abandoned in light of suitably weird phenomena in quantum mechanics.</em></p>
</blockquote>
<p>I really don't know much abou... | 5,293 |
<p>If you recorded data which represents some physical value in space (e.g. electron density) and you need to explore this dynamically in 3d (say you have a isosurface and you change the value it represents) what toolkit/program would you use?</p>
<p>Personally I used MayaVi and it has all the all the features I could... | 5,294 |
<p>Is there a way to extend or reduce the half-life of a radioactive object? Perhaps by subjecting it to more radiation or some other method.</p> | 5,295 |
<p>Can someone please explain why Indirect band gap semiconductor can not be used for LED creation. Can you also please give me some reference link for details.</p> | 5,296 |
<p>Are there any other facilities that would be capable of independantly verifying the opera result? In other words, a completely different source/detector for $\nu_{\mu}$ beams?</p>
<p>Alternatively, there might be another source that could send a beam to the same detector at Gran Sasso, with a different baseline, lo... | 5,297 |
<p>In some basic physics homework I have, it asks what the resulting vector would be for a plane traveling at x m/s in y direction that is affected by a tailwind going in z direction at w m/s. How would I calculate this? I know basic vector addition, but this has stumped me.</p> | 5,298 |
<p>In the 2011 movie <a href="http://www.imdb.com/title/tt1527186/" rel="nofollow">Melancholia</a>, a planet, also called Melancholia, enters the solar system and hits the Earth. I want to leave aside the (also unreasonable) aspect that planet "hides behind the Sun" and is undetected until it's a few weeks from Earth. ... | 5,299 |
<p>I am referring to <a href="http://pages.swcp.com/pcaskey/feynman.html" rel="nofollow">this</a>:</p>
<blockquote>
<p>Within a week I was in the cafeteria and some guy, fooling around,
throws a plate in the air. As the plate went up in the air I saw it
wobble, and I noticed the red medallion of Cornell on the p... | 5,300 |
<p>Hey so I've just learned about angular velocity and momentum and how torque changes it.</p>
<p>Looking at a <a href="http://www.youtube.com/watch?v=8H98BgRzpOM" rel="nofollow">wheel spinning around an axis, with one end being held up</a> by a rope, what causes the wheel to rotate downwards over time, and eventually... | 5,301 |
<p>Lasers are used in various industrial processes that need intense, localised, heat (3d printers and laser cutters come to mind).</p>
<p>My question is: why use lasers? There are many other (cheaper, brighter) light sources. There are even other monochromatic and coherent light sources (LEDs and mercury vapour lamps... | 5,302 |
<p>What type of magnetic fields does a Hall effect semi-conductor pick up on? AC or DC fields? How would one go about building a device that measures AC Magnetic fields?</p> | 5,303 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/6450/is-there-an-accepted-analogy-conceptual-aid-for-the-higgs-field">Is there an accepted analogy/conceptual aid for the Higgs field?</a> </p>
</blockquote>
<p>A <a href="https://www.youtube.com/watch?v... | 183 |
<p>I am given the information that a parcel of air expands adiabatically (no exchange of heat between parcel and its surroundings) to five times its original volume, and its initial temperature is 20° C. Using this information, how can I determine the amount of work done by the parcel on its surroundings?</p>
<p>I kno... | 5,304 |
<p>When EM radiation with fixed intensity and frequency strikes the metal plate, are the outgoing electrons at higher energy if the plate were charged to some potential than if the plate were simply neutral?</p> | 5,305 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/32120/why-do-we-still-not-have-an-exact-definition-for-a-kilogram">Why do we still not have an exact definition for a kilogram?</a> </p>
</blockquote>
<p>I was thinking about SI units. I found the follow... | 184 |
<p>I live on the first floor of a seven story apartment building. We are considering our hot water options and one of them is a solar-powered water heater in which water is heated in a specialised device similar to an air conditioning condenser under the sun (very common in Israel).</p>
<p>Ignoring the fact that the d... | 5,306 |
<p>I wonder if someone could tell me where my logic is going wrong here?</p>
<ol>
<li>If two particles both have definite energy, then they have indefinite position.</li>
<li>As their positions could literally be anywhere in the universe, we cannot tell them apart.</li>
<li>If two particles are indistinguishable, then... | 5,307 |
<p>According to Faraday's law of induction, volts = -Number of coils in a solenoid * change in strength of magnet / change in time. This doesn't take into account distance or speed, only time. If amps = volts / ohms, and ohms is 0, it seems like amps should be infinity. If there are infinite amps, then wouldn't the alt... | 5,308 |
<p>why streams of energy-matter are emited along the axis of rotation of a compact object?</p>
<p>(or) what is the reason that they are emited along the axis of rotation of a compact object?</p>
<p><img src="http://upload.wikimedia.org/wikipedia/commons/3/38/Protoplanetary_disk_HH-30.jpg" alt="polar jet herbig haro o... | 5,309 |
<p>I am studying Statistical Mechanics and Thermodynamics from a book that i am not sure who has written it, because of its cover is not present.</p>
<p>There is a section that i can not understand:</p>
<p>${Fj|j=1,..,N}$</p>
<p>$S= \sum_{j=1}^{N} F_{j}$</p>
<p>$<S>=< \sum_{j=1}^{N} F_{j}> = \sum_{j=1}^... | 5,310 |
<p>Which are the best introductory books for topology, algebraic geometry, manifolds etc, needed for string theory?</p> | 185 |
<p>I'm trying to understand <a href="http://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment" rel="nofollow">Stern-Gerlach experiment</a> on a computational level. Suppose we have a neutral particle with magnetic moment (e.g. a neutron), and apply an inhomogeneous magnetic field to it (let it change linearly with... | 5,311 |
<p>Assume that you fixed a speaker to an inclined pipe as well the torch. You can hear sound from the other end of the pipe, but can't see the light from other end of the pipe, why? </p>
<p><img src="http://i.stack.imgur.com/Cc7tp.png" alt="enter image description here"></p> | 186 |
<p>I know that there's a difference between relativistic rest mass. Relativistic mass is "acquired" when an object is moving at speeds comparable to the speed of light.Rest mass is the inherent mass that something has regardless of the speed its moving at.</p>
<p>When fusion happens, a certain percentage of the rest m... | 46 |
<p>In a thermodynamic turbine using air as an ideal gas, given that you have a known inlet temperature value $T_i$, a known exit pressure value $P_e$, a known inlet and exit velocity $V_i$ and $V_e$, a known value for the actual work of the turbine $w_{actual}$, and an isentropic efficiency $\eta$, how can you find the... | 5,312 |
<p>I read that magnetic fields perpendicular to a current shoot out and expand all the way to infinity.<br>
Additionally a gravitational wave, no matter how small will also expand to infinity at the velocity of light.<br>
Are these correct? If yes, why don't waves "lose" energy while "traveling"? </p>
<p><strong>Upda... | 5,313 |
<p>We know of geomagnetic flip in Earth's history by studying geologic data. Given other planets in the system also possess a magnetic field leads to the assumption that such polarity reversal may not be unique to Earth. </p>
<p>Is it possible to determine geo-astrophysically whether such geomagnetic polarity reversal... | 5,314 |
<p>I was watching a set of lectures on effective field theory and the lecturer said that you can always integrate the covariant derivative by parts due to gauge symmetry. For example, if I understand correctly, we can write:
\begin{equation}
\int {\cal D} \phi \exp \left\{ i \int d ^4 x D _\mu \phi D ^\mu \phi + \, ..... | 5,315 |
<p>I do understand why we are using the double cover, but why exactly do we make the transition to complex Lorentz transformations? Where and why are they needed? </p>
<p>To be precise:<br>
The double cover of the Lorentz group is given by $SU(2)$. Where and why do we make the transition from $SU(2)$ to $SL(2,C)$? ... | 5,316 |
<p>I'm studying for my upcoming physics course and ran across this concept - I'd love an explanation.</p> | 152 |
<p>I keep reading in the Physics World focus issue on vacuum technology about scientists creating high temperatures in the vacuums etc.</p>
<p>If heat is caused by thermal energy being radiated from particles due to their energy, then how can there be heat in a vacuum, as there are no particles present?</p> | 5,317 |
<p>I have been given the Lorentz generator
$$J_{ab}=\lambda_a \frac{\partial}{\partial \lambda^b}+a\leftrightarrow b$$</p>
<p>I know $\frac{\partial}{\partial \lambda^b}\lambda_a=\epsilon_{ba}$ and $A_a=A^b\epsilon_{ab}$ (? not sure about these two).
I want to show that this acting on a spinor bracket gives zero as t... | 5,318 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/21051/why-are-continuum-fluid-mechanics-accurate-when-constituents-are-discrete-object">Why are continuum fluid mechanics accurate when constituents are discrete objects of finite size?</a> </p>
</blockquot... | 187 |
<p>I read about the near-earth asteroid <a href="http://en.wikipedia.org/wiki/99942_Apophis" rel="nofollow">99942 Apophis</a>. It is in the first place of Potentially Hazardous Asteroids (visiting earth at an altitude of about 36,000km on 2028). But, scientists have calculated that the asteroid would impact earth on 20... | 5,319 |
<p>Just wondering, what would happen in this experiment.</p>
<p>In the experiment you would first have two entangled particles.</p>
<p>Then you fire one of the particles, lets say "Particle A", at a double slit towards a detector.</p>
<p>While in transit to the detector, what if the other entangled particle, lets ca... | 5,320 |
<p>In quantum mechanics when we talk about the wave nature of particles are we referring in fact to the wave function? Does the wave function describes the probability of finding a particle (ex: photons) at some location? So do the "waves" describe probabilities just the way in classical physics the electromagnetic wa... | 5,321 |
<p>where could i find an explanation (appart from Wikipedia) of how the Hawking's effect is obtained from quantum field theory GR and thermondynamic :D </p> | 5,322 |
<p>I recently found a problem that looked like this:</p>
<blockquote>
<p>A box sits on a horizontal wooden ramp. The coefficient of static friction between the box and the ramp is <code>.30</code>. You grab one end of the ramp and lift it up, keeping the other end of the ramp on the ground. What is the Angle between... | 5,323 |
<p>I know it's more than 70% water. But what has it got to do with earth's colour ?</p> | 5,324 |
<p>I've been trying for days, but I just can't understand why escape velocities exist. I've searched the web and even this site, and although I've read many explanations, I haven't been able to <em>truly</em> understand them. Most of the explanations I've seen involve calculus; I only know very little calculus. Could s... | 5,325 |
<p>This question is a continuation from <a href="http://physics.stackexchange.com/a/34677/10531">this one</a>.</p>
<p>A material disk have two sides, one that is reflective and another absorptive of electromagnetic radiation in the range where the background cosmic radiation spectrum is significative.</p>
<p>It is pr... | 5,326 |
<p>How far would I have to go to see a fully rounded Earth?</p>
<p>Recently, I saw a video on Youtube in which a sky diver called Felix Baumgartner ascends to $120,000$ feet (= $39$ miles) in a stratospheric balloon and make a freefall jump, rushing toward earth at supersonic speeds.<br>
I could see the full face of t... | 5,327 |
<p>I'm studying fluid mechanics in more depth during my Ph. D. and there is something related with the diffusive term that has been bothering me for a long time. Looking at the convection diffusion equation:
$$
\frac{\partial u}{\partial t} + a\cdot\nabla v - \nabla(\nu \nabla v )=f,
$$</p>
<p>and thinking in Fick's l... | 5,328 |
<p>Say you want to store hot coffee in a container surrounded by a vacuum. To remove all sources of conductive energy loss the container is suspended in the vacuum by a magnetic field and does not have a physical connection to the sides of the vacuum chamber,</p>
<p>My question is would the magnetic field be a path fo... | 5,329 |
<p>I would like to ask if someone knows the physical meaning of Boltzmann's definition of entropy. of course the formula is pretty straightforward </p>
<p>$$S=K_b\ln(Ω)$$</p>
<p>but what in the heck is the natural logarithm of micro states of the systems? I mean what does it express?</p>
<p>Maybe it is a dumb questi... | 5,330 |
<p>The background: I'm doing some simulation work involving the diffusion equation in 1D. Specifically I have some temperature profile, constant thermal conductivity and fixed temperature at each end of the system.</p>
<p>I know that we can write:</p>
<p>$$
\tau = \frac{L^2}{\kappa}
$$</p>
<p>where $\tau$ is the ch... | 5,331 |
<p>I have a working knowledge of wave-particle duality, I think. I know the de Broglie wavelength is a sort of probability of finding a particle in a specific position, and is calculated by $\lambda=\frac{h}{\vec{p}}$. I have a couple questions I'm hoping to have cleared up, though.</p>
<p>First, since $\vec{p}$ is mo... | 5,332 |
<p>I'd like to ask some questions about flipping two coins related to statistical mechanics, e.g. microcanonical distribution, phase space distribution function etc... after I rephrase the coin flipping problem into the language of statistical mechanics.</p>
<p>In probability theory, given the following problem</p>
<... | 5,333 |
<p>One of the most common gauges in QED computations are the $R_{\xi}$ gauges obtained by adding a term
\begin{equation}
-\frac{(\partial_\mu A^{\mu})^2}{2\xi}
\end{equation}
to the Lagrangian. Different choices of $\xi$ correspond to different gauges ($\xi=0$ is Landau, $\xi=1$ is Feynman etc.) The propagator for the... | 5,334 |
<p>In Minkowski spacetime, two observers, A and B, are moving at uniform speeds u and v, respectively, along different trajectories, each parallel to the y-axis of some inertial frame S. Observer A emits a photon with frequency $\nu_{A}$ that travels in the x-direction in S and is received by observer B with frequency ... | 5,335 |
<p>I am having some trouble understanding three-phase alternating current. I realize that most houses are not three-phase but single phase. Would that not mean that at some point when the flow of electricity switches direction it will stop and the electrical motor would stop under single phase? </p>
<p>Also, I also re... | 5,336 |
<p>Hi all I'm a little confused </p>
<p>So I have that in special relativity time is included as a coordinate so that in 1 spatial dimension we have 2 space time coordinates. The most basic metric is the Minkowski metric given by $\left[ {\begin{array}{*{20}{c}}
{ - 1}&0 \\
0&1
\end{array}} \right]$ ... | 5,337 |
<p>I am trying to calculate the expectation value of an infinite quantum well in one dimension (L).</p>
<p>Given:<br>
$$\phi_n = \sqrt{\frac{2}{L}}\sin\left(\frac{n\pi}{L}x\right)$$ and $$E_n=\left[\frac{\hbar^2\pi^2}{2mL^2}\right]n^2$$</p>
<p>I am trying co calculate $\langle\phi_n|\hat{T}|\phi_n\rangle$, where $\ha... | 5,338 |
<p>This question probably verges on pseudo-science and probably sounds like gibberish, so please pardon me. But I'll ask it anyway.</p>
<p>In an ideal lab experiment there is generally a separation between the system to be studied and the apparatus (environment). The two are considered 'unentangled' before the decoher... | 5,339 |
<p>In antenna technology we distinguish between nearfield and widefield. In the nearfield the electric and the magnetic fields are shifted by 90°. If you look closer you can see that there are two possibilities of this shift, 90° and -90°. </p>
<p>To explain it you have to remember what this 90° means. Let (in vacuum)... | 5,340 |
<p>I'm not familiar with any complicated physics equation, however I do understand some basics. Suppose there is two objects, both of them are moving away from each other in a 3-dimensional space, which they both have the speed of half the speed of light ($c/2$). Relative to the reference frame of object A, object B wo... | 5,341 |
<p>$$\vec{F}_\text{mag} = \frac{\mu_0 I_1 I_2}{4\pi } \oint\oint\frac{\mathrm{d}\vec{l}_1\times(\mathrm{d}\vec{l}_2\times\hat{r})}{r^2}$$</p>
<p>Is there any derivation for <a href="http://en.wikipedia.org/wiki/Amp%C3%A8re%27s_force_law" rel="nofollow">Ampère's force law</a> or is it just derived empirically?</p>
<p>... | 5,342 |
<p>Through various raids and acts of sabotage during WWII, the Allies succeeded in preventing Germany from coming into possession of large quantities of heavy water produced by the Norsk Hydro plant in Vemork Norway. These actions were carried out at considerable expense in terms of Allied troops and civilian casualti... | 5,343 |
<p>I am a chemist with a passion for astrophysics and particle physics, and one of the most marvellous things I have learned in my life is the process of stellar nucleosynthesis. It saddens me how my colleagues point to the periodic table and draw compound structures so often, yet never seem to be curious about where t... | 5,344 |
<p>When dealing with fluid mechanics of viscous fluids, both theoretically and numerically, I've always been told that the boundary condition applied at solid walls has to be a no-slip one. My teachers or textbooks never really explained why, except sometimes "well, the fluid's viscous, so it sticks to solid" which is ... | 5,345 |
<p>Today I have by accident thrown a AAA battery into a bucket of water. I fished it out of the water immediately (within 20 seconds or so) and nothing notable had happened and the battery is still full according to a battery test device. As the water should have short circuited the battery I would have expected that s... | 5,346 |
<p>The first working point-contact transistor made in 1947 by Bell Labs. I'm looking for specific dimensions, all I've been able to find is "Fits in the palm of your hand".</p> | 5,347 |
<p>To check the correlation between Hidden Variable Theory and Quantum Mechanics, Bell calculated the expectation value </p>
<p>$<\sigma_{e}(\vec a,\vec V) \sigma_{p}(\vec b,\vec V)> = \int d^n V \rho(\vec V) \sigma_{e}(\vec a,\vec V) \sigma_{p}(\vec b,\vec V)$</p>
<p>Here I am assuming that "Alice" is measurin... | 5,348 |
<p>It seems that water generally dampens sound waves. Is there any way one could attach a speaker to a body of water in such a way that the water would actually amplify some frequencies (for nearby listeners in the air, not under water)?</p>
<p>Imagine a speaker on a boat on a small creek. Listeners are on the bank.<... | 5,349 |
<p>Say you have a charged particle in a region that contains a fluid that will produce a drag force that goes as $F=-kv$ where $v$ is the speed and $k$ is some constant. The region also contains a uniform magnetic field.
Suppose you give the particle some initial velocity $v_0$ in the plane perpendicular to the magnet... | 5,350 |
<p>I asked on <a href="http://math.stackexchange.com/q/306253/11127">Math.SE</a> and was advised to try here instead.</p>
<p>I need to draw from a Maxwell-Boltzmann velocity distribution to initialise a molecular dynamics simulation. I have the <a href="http://en.wikipedia.org/wiki/Probability_density_function" rel="n... | 5,351 |
<p>I see there are different accounts on the number of physical constants. Is there any theory predicting what the number of physical constants is? I agree there should be at least one but I have no justification why there should be more than one constant. </p> | 5,352 |
<p>I have always been confused by the relationship between the <a href="https://en.wikipedia.org/wiki/Theoretical_and_experimental_justification_for_the_Schr%C3%B6dinger_equation" rel="nofollow">Schrödinger equation</a> and the <a href="https://en.wikipedia.org/wiki/Electromagnetic_wave_equation" rel="nofollow">wave eq... | 188 |
<p>I am having trouble understanding how to solve some theoretical physics problems I have come across. Specifically how to convert the Hamilton-Jacobi equation:
$$(\partial_\mu S+e A_\mu)^2=m^2$$
From Minkowski-space notation to conventional form:
$$(\frac{\partial S}{\partial t}+\frac{\alpha}{r})^2-(\nabla S)^2=m^2$$... | 5,353 |
<p>In an experiment where the type of metal,intensity of light and potential difference across a battery is kept constant at 2V the results show that an increase in wavelength, obviously in turn decreases the frequency, causes the current of the circuit to decrease (eventually to 0A).
what would be the direct cause for... | 5,354 |
<p>If you want to fly a spaceship with human passengers as close to the Sun as possible, then what effects would the spaceship have to be designed to counteract in order to keep the passengers alive and how close to the Sun could you get before there would be no way to counteract the effects ?</p> | 5,355 |
<p>The footnote (bottom) for the following paragraph from “Creative Tension: Essays On Science & Religion” by Michael Heller, has “<em>such a projection, being always “onto,” switches off all possible “short distance correlations</em>.” I’ve looked at the references and cannot see how this “switching off” process o... | 5,356 |
<p>Suppose I have two rigid bodies A and B and they are connected by a spring which is attached off-center (thus possibly causing torques). Due to the spring a force $f$ acts on A and a force $-f$ acts on B (at the respective attachment points) in direction of the spring as in Fig. 1. How can I show the conservation of... | 5,357 |
<p>I met some problem about the Virasoro operator in "old covariant quantization" in Polchinski's string theory vol I p 123.</p>
<p>It is given
$$L_0^{\rm m}=\alpha' p^2 + \alpha_{-1} \cdot \alpha_1 + \cdots \tag{4.1.11a} $$
But on p 59,
$$ L_0 = \frac{ \alpha' p^2 }{4} + \sum_{n=1}^{\infty} ( \alpha^{\mu}_{-n} \alp... | 5,358 |
<p>I am confused with adiabatic expansions. I have a homework problem wherein 2.75 moles of an ideal gas at 375 K expands adiabatically with an initial pressure of 4.75 bar and a final pressure of 1.00 bar with a C(p,m)=5R/2. I need to calculate the work done by the expansion. </p>
<p>Where do I start (I don't want j... | 5,359 |
<p>Basically, I would really like it if somebody just explained to me what is going on here. Please use any physics lingo you feel is necessary, but explain what you mean. I am just having trouble finding a nice explanation without having to know tons of physics. ( I have a fifth grade physics level)</p>
<p>So the ref... | 5,360 |
<p>I was doing some exercises the other day, when I came across this question in my book:</p>
<blockquote>
<p><em>A proton weighs about 1.66 x 10<sup>-24</sup> g and has a diameter of
about 10<sup>-15</sup> m. What is its density in g/cm<sup>3</sup>?</em></p>
</blockquote>
<p>As you can see, a really simple and ... | 5,361 |
<p>In biology, it is stated that more surface area and less volume helps for keeping bodies cool. How can one explain this phenomena in terms of physics?</p> | 5,362 |
<p>I hope this question is simple and can be quickly cleared up.</p>
<p>In a 1D conservative dynamical system, I've always been taught that the potential function is the function $V(x)$ such that:</p>
<p>$$F=-\frac{dV}{dx}$$</p>
<p>That makes sense to me, simply derived from the definitions of work and conservation ... | 5,363 |
<p>Recently I have discovered the method of constructing of GR from massless field with helicity 2 theory. It is considered <a href="http://arxiv.org/abs/gr-qc/0411023" rel="nofollow">here</a>, in an article "Self-Interaction and Gauge Invariance" written by Deser S. </p>
<p>By the few words, the idea of the method is... | 5,364 |
<p>Now this might be a silly question but it's actually bugging me, this one might be easier to understand if you have kids that watch (or used to watch) <a href="http://en.wikipedia.org/wiki/Peppa_Pig">Peppa Pig</a>. In one of the episodes, about shadows, the kids try to run away from their shadows, they try to move f... | 993 |
<p>Say I use the metric signature $(-+++)$. Then
$\partial_a=(\partial_0,\partial_i)=(-\partial^0,\partial^i)$,
but
$\partial^a=(\partial^0,\partial^i)=(-\partial_0,\partial_i)$.</p>
<p>The same goes for $p^a$ and $p_a$, I suppose. I know that we need to contract, say, $x^a$ with $p_a$ to give $-x^0p_0+x^ip_i$. So my ... | 5,365 |
<p>First of all, what is the mathematical relationship between measured linewidth (usually in units of magnetic field) and spin relaxation time? I see papers talk about spin relaxation times in terms of linewidths but I have no idea how to correlate the size of the linewidth to an actual time. </p>
<p>Second, is the m... | 5,366 |
<p>It is stated that "the formula for the energy stored in the magnetic field is: $$E = \left(\frac{1}{2}\right)(LI)^2$$ and the energy stored in the magnetic field is equal to the <em>work</em> done to produce the current.</p>
<p>What is the formula for <strong><em>work</em></strong> , that shows they are equal?
I... | 5,367 |
<p>My question is the following:</p>
<p><em>What is the shape of the rope which holds a kite flying?</em> (Steady state.)</p>
<p>I am not a physicist (I am a mathematician), so I can not work the physics part of the question. </p>
<p>My guess is that it should be the same as catenary (hyperbolic cosine) </p> | 5,368 |
<p>I am currently a first year Ph.D. student in Applied Math. As an undergrad, I did not have the opportunity to take a calculus based physics course and the one I took was a general physics course where emphasis was on the concepts (most of which I already forgot).</p>
<p>Given my mathematical background, I honestly ... | 5,369 |
<p>Is there a way to find a solution for the positions (as a function of time) of multiple free bodies that push and pull on each other?</p>
<p>Say for instance I have a collection of cells which can individually contract or expand, and are joined to one another. For simplicity, I'm assuming two dimensions; no gravity... | 5,370 |
<p>We are looking at White Dwarfs in Quantum and specifically how they are stabilized between gravitation pressure inwards balanced by the Fermi pressure due to electrons. </p>
<blockquote>
<p>To start the problem off we are asked to calculate the inward pressure due to gravity in terms of the total gravitational en... | 5,371 |
<p>There are plenty of formulas that use gravity acceleration of Earth. This is represented with the symbol $g$. In my school work (I am a high school student) we usually take it as $g= 9,8 \,\text m/\text s^2$. </p>
<p>This thing is obviously a number that is only usable on Earth. What I want to know is that, what if... | 5,372 |
<p>Q3 from a mechanics exam past paper:<img src="http://i.stack.imgur.com/95nD6.png" alt="enter image description here"></p>
<p>I can do parts i) and ii) but for iii) in finding the angular acceleration, i used $C=I\alpha$, where $C$ is the applied couple or torque, $I$ is the moment of inertia for the lamina about A ... | 5,373 |
<p>I am a programmer and I want to pursue the Game Design field. From talking to my teachers about it, they said that physics plays a major role. My Question is: How does physics transfer to a virtual computer based environment. Is it fundamentally the same or can you create a world that defies our rules.</p> | 5,374 |
<p>I am reading section 3.2.1 of Griffiths 3ed which explains how to calculate potential using first uniqueness theorem.</p>
<p>Griffiths/3.2.1
<img src="https://i.imgur.com/hTmCHFX.png" alt=""></p>
<p>Griffiths/First Uniqeness theorem (Its corollary actually)</p>
<h2><img src="https://i.imgur.com/JlGMUEl.png" alt="... | 5,375 |
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