question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>Imagine a black hole that is fast-approaching its final exponential throws of Hawking evaporation.</p>
<p>Presumably, at all points in this end process there there will remain a region that identifiably remains "the black hole" until the the very end, as opposed to huge swarm of fundamental particles that is being ... | g13786 | [
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<p>Consider watching an alien space ship at Alpha Centauri (at 4.5 light years away) through a telescope from earth. This space ship turns towards us, and starts travelling toward us at what appears to be a velocity of exactly 2 million kilometers per second, when measuring the distance every hour.</p>
<p>The space sh... | g13787 | [
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<p>Electron and a positron are headed towards each other with same kinetic energies - observed from <em>center of mass</em> (COM) system. After the colission two photons are emitted and are headed in opposite directions with the same momentum. hat are photon wavelengths?</p>
<p>In almost every pair production problem ... | g13788 | [
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<p>I am a graduate physics student. I have started learning QFT. As a project my professor has asked me to take up and learn <a href="http://www.google.com/search?hl=en&as_q=schwinger+source+theory"><strong>Source Theory</strong></a>, seems an alternative to regular QFT. How exactly is this formulation different? I... | g13789 | [
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<p>I play the flute as a hobby, and I've noticed that when playing middle D or E flat, one can interrupt the air column by releasing a certain key (which is near the middle of the air column), and yet have no effect on the pitch (though the quality changes for the better).</p>
<p>I'll be putting a few diagrams here, s... | g13790 | [
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<p>Why does the <a href="http://en.wikipedia.org/wiki/Cosmic_censorship_hypothesis" rel="nofollow">cosmic censorship conjecture</a> hold so well?</p>
<p>Penrose proposed spacelike singularities and closed timelike curves are always hidden behind event horizons in general relativity. His conjecture appears to hold pret... | g13791 | [
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<p>Let's restrict to the case of spin-1/2 system. As we know, a spin-liquid (SL) state is the ground state of a lattice spin Hamiltonian with no spontaneous broken symmetries (sometime it may spontaneously break time-reversal symmetry and is called a chiral SL), where two <em>essential</em> symmetries of a SL state are... | g13792 | [
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<p>I spent a long time being confused by the Heisenberg uncertainty principle in my quantum chemistry class.</p>
<p>It is frequently stated that the "position and momentum of a particle cannot be simultaneously known to arbitrary precision" (or any other observables $[A, B] \neq 0$).</p>
<p>This made no sense to me -... | g13793 | [
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<p>I'm reading Zwiebach's First Course in String Theory. At present I'm learning about string coupling. Zwiebach says it's possible to prove that $g_o^2=g_c$ where $g_o,\ g_c$ are open and closed string coupling constants respectively. He claims this is due to certain topological properties of world sheets.</p>
<p>Why... | g13794 | [
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<p>Let $\left(|\uparrow\rangle,|\downarrow\rangle\right)$ and $\left(|\nearrow\rangle,|\swarrow\rangle\right)$ be two bases of the $2$-dimensional Hilbert space $H$.</p>
<p>Can an experiment distinguish between $\frac 1{\sqrt 2} \left(|\uparrow\downarrow\rangle - |\downarrow\uparrow\rangle\right)$ and $\frac 1{\sqrt 2... | g13795 | [
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<p>This may be a noob question but I've tried searching about this and haven't been able to put things into the context of what I've been studying.</p>
<p>(Dot means the usual derivative w.r.t. time)</p>
<p>If $c$ and $\bar{c}$ are independent anti-commuting variables, I want to confirm a few properties: first, will ... | g13796 | [
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<p>As you can see this image <a href="http://i.stack.imgur.com/YOt8C.jpg" rel="nofollow">http://i.stack.imgur.com/YOt8C.jpg</a> and other galaxy images, the centers generally much brighter. </p>
<p>Why is that? </p>
<p>Is there a very big star? A very big gravitational field?</p> | g13797 | [
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<p>My question is whether or not it would be possible to create an entangled state between two photons that do not share the same initial photon source and their respective sources are separated by an arbitrary distance in space. This is a curiosity of mine and I have a very basic understanding of this phenomena at pr... | g13798 | [
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<p>I tried calculating the downforce of a <a href="http://en.wikipedia.org/wiki/Spoiler_%28automotive%29" rel="nofollow">spoiler</a> I need for a robot. I used this formula.</p>
<p>$$F = \frac{1}{2}Ac_a\rho v^2$$</p>
<p>Where $\rho$ is air density, $v$ is the speed of my robot, $c_a$ is the lift coefficient and $A$ i... | g13799 | [
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<p>In the review of Slanksy "Group theory for unified model building" in chapter 6:
How do one relate the projected Dynkin diagrams from for example $\overline{5}+10$ of $su(5)$ to the corresponding representation in of the subgroup $su(2)\times su(3)$?</p>
<p>An example is
$P( 0 0 1)=(0)(01)$, how do one see that th... | g13800 | [
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<p>Eliezer Yudkowsky wrote <a href="http://wiki.lesswrong.com/wiki/The_Quantum_Physics_Sequence">an introduction to quantum physics</a> from a strictly realist standpoint. However, he has no qualifications in the subject and it is not his specialty. Does it paint an accurate picture overall? What mistaken ideas about... | g13801 | [
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<p>I was doing kinematics these days, projectile motion. And I did the basic concepts: range, max height, time of flight, projectile on inclined plane. I am curious to know how we would solve a question if it asks us to find the distance not range traveled by the particle undergoing projectile motion. Is it by finding ... | g13802 | [
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<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/20333/speed-of-a-fly-inside-a-car">Speed of a fly inside a car</a> </p>
</blockquote>
<p>Just a conceptual question: If a flying bee is inside a speeding vehicle, will it have to "fly" just as fast as th... | g432 | [
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<p>I am performing an experiment where I'm measuring two variables, say $x$ and $y$, but I'm actually interested in a third variable which I calculate from those two,
$$z=f(x,y).$$
In my experiment, of course, both $x$ and $y$ have experimental uncertainties, which are given by the resolution of my measurement apparatu... | g13803 | [
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<p>Encountered a problem that involves impulse while studying for my exam and I'm not sure how to even approach it. I know that momentum is conserved, but I'm not sure how to relate that to avg force. Maybe someone can help point me in the right direction? I know that it's in quadrant III, through intuition, but I can'... | g13804 | [
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<p><img src="http://i.stack.imgur.com/B1KNO.png" alt="enter image description here"></p>
<p>The book's logic is that there is no induced EMF because flux is constant as it passes through the magnetic field. Which makes sense, but this seems counter-intuitive to what I previously learned. </p>
<p>If i'm not mistaken, ... | g13805 | [
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<p>I'm having difficulties with Neumann boundary conditions in Navier-Cauchy equations (a.k.a. the <a href="http://en.wikipedia.org/wiki/Linear_elasticity#Elastostatics" rel="nofollow">elastostatic equations</a>). The trouble is that if I rotate a body then Neumann boundary condition should be satisfied with zero force... | g13806 | [
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<p>I was thinking about the <a href="https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis" rel="nofollow">Mathematical universe hypothesis</a> and a natural question popped into my mind:</p>
<p><em>Assuming that the universe (by universe I mean the complete physical reality) is really isomorphic to some conc... | g13807 | [
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<p><em>The Revelation is one of the fastest and most exhilarating extreme rides in the world! Designed like a giant airplane propeller, the Revelation holds two to four riders comfortably in the cockpit seats at each end of its 160-foot arm. Riders take off quickly spinning up to 100 kph and experience the same intense... | g13808 | [
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<p>We know that energy of quarks inside the proton can not be exactly fixed because if it,the 'proton decay' must not be exist. My question is if the energy of quarks inside the proton is not exactly fixed than the mass of the proton must be fluctuate because 99% of the proton mass is due to the kinetic energy of the q... | g13809 | [
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<p>Will the whole liquid boil quicker on a stove-top if it stirred versus if not stirred? What should be the frequency of stirring to get to boiling point quickly? please explain your hypotheses with experiments(or links to) preferrably</p> | g13810 | [
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<p>I may just be being very dense (no pun intended) but i'm reading up on gravitational lensing and it seems to require a notion of density (e.g. see <a href="http://en.wikipedia.org/wiki/Gravitational_lensing_formalism" rel="nofollow">here</a>)</p>
<p>I'm working on a question involving light bending around a point m... | g13811 | [
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<p>Suppose I have four wires, and I tensor product them together</p>
<p>$A \otimes B \otimes C \otimes D$</p>
<p>I pass $A \otimes B$ through a spatial beam splitter </p>
<p>$Spl: A \otimes B \rightarrow A^\prime \otimes B^\prime$ </p>
<p>and I pass $C \otimes D$ through a polarizing beam splitter </p>
<p>$Pspl :... | g13812 | [
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<p>I came across this abstract, and I am curious as to what the ATLAS Team has actually discovered:</p>
<blockquote>
<p>Abstract Motivated by the result of the Higgs boson candidates at LEP with a mass of about 115~GeV/c2, the observation given in ATLAS note ATL-COM-PHYS-2010-935 (November 18, 2010) and the publicat... | g13813 | [
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<p>I have on several occasions wondered how one might proceed in order to sample large subcritical Bernoulli bond-percolation clusters, say on the square lattice.</p>
<p>More precisely, let's consider the lattice $\mathbb{Z}^2$, and open each edge independently with probability $p<p_c=0.5$. I am interested in the e... | g13814 | [
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<p>Just a quick question, same as in the title. I'm trying to understand stable D-branes.</p> | g13815 | [
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-0.08028415590524673,
-0.07468874752521515,
0.03147326409816742,
0.01696217991411686,
-0.0... |
<p>There is a class of observables in QFT (event shapes, parton density
functions, light-cone distribution amplitudes) whose the renormalization-group
(RG) evolution takes the form of an integro-differential equation:
$$
\mu\partial_{\mu}f\left( x,\mu\right) =\int\mathrm{d}x^{\prime}\gamma\left(
x,x^{\prime},\mu\righ... | g13816 | [
0.010801130905747414,
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0.060930345207452774,
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0.0... |
<p>Is there accepted nomenclature for the star around which a particular exoplanet orbits? Meaning, if I were to say "The exoplanet blah blah blah's (noun)" what noun would I put there? Sun? Star? Designated Gravity Buddy? Something else?</p>
<p>As I understand it, this would not come up often currently in the real w... | g13817 | [
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<p>The possibility though remote, is intriguing as we may be able in the future to actually "see" our own planet's history.
Though sounding science fiction, if we are able to detect bodies in space that are able of reflecting light emitted from our planet earth, using amplification systems and filters,this may - if pos... | g13818 | [
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<p>This form is taken from a talk by Seiberg to which I was listening to, </p>
<p>Take the Kahler potential ($K$) and the supersymmetric potential ($W$) as, </p>
<p>$K = \vert X\vert ^2 + \vert \phi _1 \vert ^2 + \vert \phi_2\vert ^2 $ </p>
<p>$W = fX + m\phi_1 \phi_2 + \frac{h}{2}X\phi_1 ^2 $ </p>
<ul>
<li>This n... | g13819 | [
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<p>Specifically, how can decoherence explain the appearance of flecks of metallic silver on a photographic plate when exposed to the very weak light of a distant star?</p>
<p>EDIT: Perhaps the advocates of decoherence need some context for this question. There is a certain definite quantity of energy on the order of o... | g13820 | [
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<p>Facts agreed on by most Physicists -</p>
<p>GR: One can't apply Noether's theorem to argue there is a conserved energy.
QFT: One can apply Noether's theorem to argue there is a conserved energy.
String Theory: A mathematically consistent quantum theory of gravity.</p>
<p>Conclusion -</p>
<p>If one can apply Noeth... | g13821 | [
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0.004... |
<p>I am teaching a multivariable calculus course and we are starting to go over surface integrals. I am a math professor with little knowledge of physics. At one point the book discusses fluid flow. It is trying to convince me that the surface integral of a velocity field is the flow of the fluid across the surface.... | g13822 | [
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<p>The following question is about chapter 2 of Sakurai's <em>Modern Quantum Mechanics</em>. I wish I could link to the Google book, but it doesn't seem to have a satisfactory preview to be able to read the section I'm talking about, so I'll do my best to write out the part I'm talking about...</p>
<p>In the section a... | g13823 | [
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0.033... |
<p>Does someone know explain me how to identify the multipoles magnetic terms of the multipolar expansion (Dipole, quadrupole, etc) in spherical harmonics?</p> | g13824 | [
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<p>What we regard as TIME is just a way of measuring duration for various phenomena. Like a ruler is a measuring device for measuring length ( or breadth or width). Saying Time is an illusion is like saying the measuring 'ability' of a ruler is an illusion. Is a measure of length, width, or breadth just an illusion? Wh... | g13825 | [
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<p>The thing with <a href="http://en.wikipedia.org/wiki/Global_warming" rel="nofollow">global warming</a> is that it absorbs infrared (IR) radiation from the planet and reradiates much of it back to the planet (whereas the Sun's peak flux is in the visible region, that is unaffected by CO<sub>2</sub>).</p>
<p>But with... | g13826 | [
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-0.03928089... |
<p>How big is the solar system? </p>
<p>By "big", I guess I mean "wide", i.e. how far away from the Sun is the farthest object that is considered part of the Solar System?</p>
<p>I've checked Wikipedia's pages on the <a href="http://en.wikipedia.org/wiki/Solar_System">Solar System</a>, as well as <a href="http://en.w... | g13827 | [
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<p>From what I understand the asteroid belt never formed into a planet because Jupiter threw off the gravity of all the small objects and they never could gather to form a planet. As described here: <a href="http://astronomy.stackexchange.com/questions/861/why-did-the-asteroid-belt-between-mars-and-jupiter-form-as-it-... | g13828 | [
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-0... |
<p>Is it possible to extract the molecular kinetic energy from a system directly (without the use of a heat engine / temperature gradient) and convert that to
another form of energy, such as electricity, or perform useful work?</p>
<p>Similar questions have been asked before:</p>
<ul>
<li><a href="http://physics.stac... | g13829 | [
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0.... |
<p>Imagine we could build a channel (like those cool <a href="http://www.amusingplanet.com/2012/09/3-most-impressive-water-bridges-around.html" rel="nofollow">ones</a> in some countries) but very, very long, parallel to a meridian (ie South-North direction).. The channel would contain water, which would permanently flo... | g13830 | [
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<p>In EM radiation, the magnetic field is $ 3*10^8$ times smaller than the electric field, but is it valid to say it's "weaker". These fields have different units, so I don't think you can compare them, but even so it seems like we only interact with the electric field of EM radiation, not the magnetic field. Why is th... | g13831 | [
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-... |
<p>This is motivated by <a href="http://puzzling.stackexchange.com/questions/1959/candle-timing-measurement">this question in the Puzzling.SE beta</a> about measuring 90 minutes of time using two candles that burn for one hour. (Feel free to read up on that before I spoil the puzzle!)</p>
<p>The solution given was to ... | g13832 | [
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0.03... |
<p>I'm struggling to write Schrodinger equation with a central potential in Atomic unit.</p>
<p>$$
\frac{\hbar^2}{2\mu}\Big(\frac{d^2}{dr^2}+\frac{2}{r}\frac{d}{dr}-\frac{l(l+1)}{r^2}\Big)R(r)+\Big(E-V-\frac{e^2}{4\pi \epsilon_0 \epsilon r}\Big)R(r)=0
$$
Taking Effective Bohr radius $a_0$ as length scale and effective... | g13833 | [
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<p>If we have an Atwood machine where masses $m_A$ and $m_B$ rest on the ground, then we apply an upwards force $F$ to the Atwoods machine.</p>
<p>What is the acceleration of the blocks when $F=124N$, $m_A = 20$kg, $m_B = 10kg$?</p>
<p>I am confused to about this problem,because I am not sure how this upward force $F... | g13834 | [
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0.0070792450569570065,
0.0252... |
<p>Typically in particle physics books (not in QFT books!) I have often seen this statement that the potential between a heavy quark and its anti-quark can be "empirically" represented as $V(r) = -\frac{\alpha_s}{r} + br$ where $\alpha_s \sim \frac{\hbar c}{2}$ and $b \sim \frac{0.18}{\hbar c} GeV^2$.</p>
<ul>
<li>Is... | g13835 | [
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0.006074046716094017,
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<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/17850/uniqueness-of-eigenvector-representation-in-a-complete-set-of-compatible-observa">Uniqueness of eigenvector representation in a complete set of compatible observables</a> </p>
</blockquote>
<p>Saku... | g433 | [
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0.0... |
<p>This follows to some extent from <a href="http://theoreticalphysics.stackexchange.com/questions/649/does-kaluza-klein-theory-successfully-unify-gr-and-em-why-cant-it-be-extended">a question I asked previously</a> about the flaws of Kaluza-Klein theories.</p>
<p>It appears to me that Kaluza-Klein theories attach add... | g13836 | [
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<p>I have some confusion about orbitals in multielectron atoms.</p>
<p>Let's say we consider an atom (Lithium, for example, $1s^2\, 2p^1$) and that the state of the last electron is [n=2, l=1, ml=0, s=1/2, ms=1/2]. Its wave equation should then be $$\psi(r,\theta,\phi)=R_{nl}(r)Y_{lm_l}(\theta,\phi)\chi_s(m_s)$$</p>
... | g13837 | [
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<ul>
<li><p>Whenever Feynman rules are stated they are always without any mention of the helicities - this I find to be very confusing. How does one introduce and account for that? </p></li>
<li><p>Is there an intuitive/simple argument for why massless particles should have "helicities" (and not polarizations) and they... | g13838 | [
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<p>So I've calculated the answer to this problem, but my answer is different from my book's, so I'm trying to find the error. A spring with coefficient $k=600N/m$ launches a mass of $1.2kg$ from an initial displacement of $0.15m$. It slides along a frictionless surface and then goes up an inclined plane with coeffici... | g13839 | [
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<p>On June 2, 2011, a new supernova was identified by an amateur French astronomer in M51.</p>
<p>What size telescope aperture would one need to have any chance of seeing this? Given the right sized scope, how would I best identify which point of light is the supernova?</p> | g13840 | [
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... |
<p>I'm no physicist apart from basic 3d web animation, I'm just curious and please feel free to correct my misuse of terms or inadequate speculations. </p>
<p>I've been reading a lot on gyroscopes and aerodynamics and the various concepts of lift, drag, etc. I understand how a rocket utilizes lift and drag and how muc... | g13841 | [
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... |
<p>From what I know, the Moon is accelerating away from the Earth. Do we know when it will reach escape velocity? How do we calculate this?</p> | g317 | [
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<p>I've read various ideas about why the moon looks larger on the horizon. The most reasonable one in my opinion is that it is due to how our brain calculates (perceives) distance, with objects high above the horizon being generally further away than objects closer to the horizon.</p>
<p>But every once in a while, the... | g106 | [
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<p>For a Fermi liquid, the Fermi momentum is determined by the singularity of the Green's function at $\omega=0$, i.e., $G(\omega=0,{\bf k}={\bf k}_F)\to\infty$.</p>
<p>Suppose due to an external field or disorder, the charge density (or the chemical potential) is not uniform, i.e., it depends on the position. Now the... | g13842 | [
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0.014111417345702648,
0.01837335340678692,
-0.02... |
<p>A water rocket works like this: there is a circular slot of area $A_1$ at the bottom centre of a cylinder of cross-sectional area $A_0$ and height $L$ that is filled with water to an initial height $h_0$. This slot will fall away during launch. The water has pushed all of the air that was originally in the cylinder ... | g13843 | [
0.03715602308511734,
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0.006888154894113541,
0.023743346333503723,
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0.016388149932026863,
0.012195080518722534,
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0... |
<p>when we approach a charged rod (+) to a neutral metal rod ( not touching) a number of electrons to that side ( lets call it side B) negating the effect of the introduced electric field, reaching equilibrium.
But what about the electrons that where already on that side , will there presence affect the equilibrium , i... | g13844 | [
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<p>I have a mechanical system and I need to model in differential equations</p>
<p><img src="http://i.stack.imgur.com/BHbsD.png" alt="The Problem"></p>
<p>I tried to model the mass $M_1$ and got this</p>
<p>$m_1\displaystyle\frac{d^2\,x}{dt^2}=F-K_1(x_2 - x_1)-Ba_1\displaystyle\frac{dx}{dt}.$</p>
<p>$F$ is the for... | g13845 | [
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0.020096106454730034,
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0.05088299512863159,
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0.002901268657296896,
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-0.07312384247779846,
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-0.0016686675371602178,
... |
<p>In quantum field theory, we can calculate the effective potential of a scalar field. My question is whether the second derivative of the effective potential always represents the mass square of the particle? Why or why not?
$$
m_{eff}^{2}=\frac{\partial^{2}V_{eff}(\phi)}{\partial\phi^{2}}
$$
For example, consider a ... | g13846 | [
0.019693218171596527,
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0.04912571609020233,
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0.038173429667949677,
0.013454527594149113,
0.0086... |
<p>For an electric monopole, its potential scales with $\frac{1}{r}$, where $r$ is the distance from the point of interest to the charge. However, for a dipole, its potential scales with $\frac{1}{r^2}$.</p>
<p>I understand how the latter is derived. But is there an intuitive explanation on why the dipole potential en... | g13847 | [
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0.0... |
<p>Based on my personal observations, newer windmills seem to have three blades while older ones tend to have four or even more. <a href="http://physics.stackexchange.com/questions/11194/wind-generators-why-so-few-blades">This question</a> has excellent discussion on my three is an optimal number. But what changed? For... | g13848 | [
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0.007639772724360228,
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0.0... |
<p>This is for a class on special relativity I am to give to some school children.</p>
<blockquote>
<p>Moe is moving at 0.9c. At the instant he passed Joe he emits a flash of light. One microsecond later, Joe(at rest) sees that Moe has traveled 270m and the light flash has traveled 300m. That is, the distance betwee... | g13849 | [
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0.08811677247285843,
0.044540416449308395,
0.02030780538916588,
-0.04429604858160019,
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0.03125108778476715,
0.04649448022246361,
0.0070... |
<p>I still do not understand what a <a href="http://en.wikipedia.org/wiki/Resonance_%28particle_physics%29">resonance</a> precisely is. Is it exactly the same as a particle? Or only an excited state?</p>
<p>And why does it make a peak in some diagrams? And which diagrams?</p> | g13850 | [
0.04205917939543724,
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-0.046214... |
<p>I can find many <a href="http://emweb.unl.edu/NEGAHBAN/Em325/18-Pressure-vessels/Pressure%20vessels.htm" rel="nofollow">references</a> that give the stress in the walls of a pressure vessel for spheres and tubes, but they all seem to be limited to a thin-wall approximation. I'll limit my writing here to spherical v... | g13851 | [
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0.029422104358673096,
-... |
<p>I was trying to unlock my car, but I was out of range. A friend of mine said that I have to hold the transmitter next to my head. It worked, so I tried the following later that day:</p>
<ul>
<li>Walked away from the car until I was out of range</li>
<li>Put key next to my head (it worked)</li>
<li>Put key on my che... | g13852 | [
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<p>My professor today in the class made us a question:
"Lets say we have a teapot with water in it.The water is hot.Now we want to cool the water.
Will it cool faster if we put an ice cube above the teapot or under the teapot."</p>
<p>My answer was the it will cool faster if we put the ice cube above it because the wa... | g13853 | [
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0.031278... |
<p>There are few <a href="http://physics.stackexchange.com/q/5456/2451">questions</a> in Phys.SE concerning the speed of gravity, and the answers are traditionally that the speed of gravity equals to the speed of light. But in that case I have three more specific questions which I didn’t find in those discussions, so l... | g13854 | [
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0.001497705583460629,
0.07059868425130844,
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<p>I am working on a project involving <a href="http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street" rel="nofollow">Von Karman vortices</a> coming off of a mountain. I was able to calculate the shedding frequency (thanks to tpg2114 in a prior question), but now find it necessary to calculate the wind speed of t... | g13855 | [
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<p>I am going through Ramond's 1971 paper Dual Theory for Free Fermions Phys Rev D<b>3</b> 10, 2415 where he first attempts to introduce fermions into the conventional dual resonance model.</p>
<p>I get the 'gist' of what he's doing: he draws an analogy of the bosonic oscillators satisfying the Klein-Gordon equation, ... | g13856 | [
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0.0229... |
<p>Purely hypothetical since any kind of testing in atmosphere/space is banned by international legislation/agreement. </p>
<p>The humans have already bombed Luna so ... what could be expected to happen on Saturn if a hydrogen bomb were to explode in it's atmosphere? Would the explosion set the planet's atmosphere abl... | g13857 | [
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<p>I was asked to determine the shear forces on 12" diameter table legs supporting a few hundred pounds of static weight.</p>
<p>How does one calculate or determine the potential force on the legs and/or potential collapse of the legs given an earthquake? The legs are not secured to the flow.</p>
<p>Where can I find... | g13858 | [
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<p>When looking at the night sky, we see lots of stars. Several places tell you that the light of those stars has traveled to many light years to reach Earth and there may be others where light has not made it here yet. How can this be?</p>
<p>Assume we accept the Big Bang Theory that suggests the universe began in ... | g13859 | [
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0.... |
<blockquote>
<p><em>What force per square meter pushes 2 infinite planes charged positively when their charge density is $0.3 \, \mu C/\mathrm{m}^2$</em></p>
</blockquote>
<hr>
<blockquote>
<p><em>Second part is - using Gauss's theorem derive the equation for electric field current between infinite electrified pl... | g13860 | [
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0.048772167414426804,
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-0.04286769777536392,
... |
<p>I'm getting crazy with this problem and I think that it's pretty simple. </p>
<blockquote>
<p>An helicopter's helix is spinning at initial speed $w_0=200\ rpm$, all
of a sudden the motor stops and it decreases its velocity with a not
constant acceleration of $\alpha=-0,01\cdot w \frac{rad}{s^2} $. </p>
... | g13861 | [
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0.034585606306791306,
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0.0556817501783371,
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0.019... |
<p><em>James P. Sethna. Statistical Mechanics. Exercise 5.2:</em></p>
<blockquote>
<p>What prevents a Maxwellian demon from using
an atom in an unknown state to extract work?
The demon must first measure which side of the
box the atom is on. Early workers suggested
that there must be a minimum energy cost to... | g13862 | [
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-0.027690598741173744,
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... |
<p>This comes from a <a href="http://www.smbc-comics.com/index.php?db=comics&id=2856" rel="nofollow">Saturday Morning Breakfast Cereal (SMBC) comic</a> with a joke answer. The problem states:</p>
<blockquote>
<p><em>A 5 kilogram ball is shot directly right at 20 meters per second from a height of 10 meters. The ... | g13863 | [
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0.037646666169166565,
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0.04951682314276695,
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0.003833112074062228,
-0.009466351941227913,
0.0... |
<p>Do object reflect radio waves just like light waves? I don't know much about their use but can anyone explain how we use this reflection? </p> | g13864 | [
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<p>I have seen it stated but not explained that consistency requires you to couple massless fields to gravity using the conformal coupling, so that $trT_{\mu \nu}=0$. What is the reason for this?</p> | g13865 | [
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0.0040711574256420135,
... |
<p>When calculating the energy difference between the normal and the superconducting state in a superconductor at zero magnetic field, the result is as follows:</p>
<p><img src="http://i.stack.imgur.com/tIq7w.png" alt="enter image description here"></p>
<p>Now I'm quite confident of this result, as it is the same as ... | g13866 | [
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-0.07096196711063385,
0.0... |
<p>Physically, what is the role of a batter in baseball?</p>
<p>My question is inspired by <a href="http://physics.stackexchange.com/questions/18222/how-does-the-speed-of-an-incoming-pitch-affect-the-speed-of-a-baseball-after-it" title="How does the speed of an incoming pitch affect the speed of a baseball after it's ... | g13867 | [
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0.0375380702316761,
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0.07487712055444717,
0.022249873727560043,
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-0.03687835857272148,
-0.025959458202123642,
0.013885758817195892,
0.0313461609184742,
-0.01... |
<p>Why don't electrons accelerate in a circuit? If there is a potential difference and the electron moves in the circuit a force is exerted on it. Why does that force produce no acceleration, keeping the current constant?</p>
<p>If we add a resistor to a circuit why is the resistance 'spread' over the circuit? Is it b... | g13868 | [
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0.09778296947479248,
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0.03708783537149429,
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0.029723597690463066,
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0.0366... |
<p>To the best of my knowledge, if you double the cross sectional area of a wire you double the maximum weight it can support before breaking. But what if you use two wires of the original cross sectional area and tie them together? Can it still support twice the weight? Are there any new forces involved in this scena... | g13869 | [
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0.031866006553173065,
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... |
<p>Here I refer to a particular book <em>Molecular Quantum Mechanics</em> by Peter W. Atkins and Ronald S. Friedman, but similar derivation could be found in many other texts.</p>
<p>So, when obtaining the explicit form of the Fock matrix elements for <a href="http://www.google.com/search?as_epq=restricted+hartree+foc... | g13870 | [
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0.0166945680975914,
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0.015869947150349617,
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<p>I earlier had a doubt based on ray theory that light must reflect and so all wavelengths should propagate through SMF. This Q/A
<a href="http://physics.stackexchange.com/questions/119803/single-mode-fibers-and-ray-theory-of-light">Single-mode fibers and ray-theory of light</a>
does specify that we have to consider w... | g13871 | [
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<p>I've been browsing some Digital Holography papers these days, and have come across this fundamental question.</p>
<p>When you reconstruct the complex amplitude for the object image, you use e.g. Fresnel Transform to simulate diffraction.</p>
<p>The thing is, one of the parameters in this process is the distance, d... | g13872 | [
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0.08397259563207626,
-0.... |
<p>This question is inspired by <a href="http://physics.stackexchange.com/a/69544/26076">this answer</a>, which cites <a href="http://en.wikipedia.org/wiki/Gravitoelectromagnetism">Gravitoelectromagnetism</a> (GEM) as a valid approximation to the Einstein Field Equations (EFE).</p>
<p>The wonted presentation of gravi... | g13873 | [
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<p>Is it meaningful to ask whether general relativity is holonomic or nonholonomic, and if so, which is it? If not, then does the question become meaningful if, rather than the full dynamics of the spacetime itself, we consider only the dynamics of test particles in a fixed globally hyperbolic spacetime?</p>
<p>For th... | g13874 | [
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<p>I've read a number of the helpful Q&As on photons that mention the mass/mass-less issue. Do I understand correctly that the idea of mass-less (a rest mass of 0) may be just a convention to make the equations work?</p>
<p>From a layperson's view, it's difficult to understand how a particle of light (photon) can ... | g195 | [
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<p>How much time would it take for a traveler (in traveler's perspective) to reach a star at distance $d$, if it accelerates at 9.8 m/s² (for a comfortable travel)?</p>
<p>I understand that $V = V_0 + a\times t$ and such related high-school physics, and I know there is something called Lorentz transformations that I c... | g713 | [
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<blockquote>
<p>Calculate molal depression constant of water if latent heat of fusion of ice at $0^\circ \text C$ is $80 \text{ cal/g}$.</p>
</blockquote>
<p>All I could find was the equation $K_f=\frac{0.002T^2}{L_f(\text{in cal/g})}$ at <a href="http://www.sciencehq.com/chemistry/depression-of-freezing-point.html"... | g13875 | [
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<p>I have a 500 gram neodymium magnet. It is stuck to my fridge. With the constant pull of gravity trying to pull it off how long would it stay attached to fridge? For arguments sake lets say my fridge is very sturdy and will never need to be replaced!</p>
<p>If my fridge was very sturdy would it still be attached whe... | g13876 | [
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<p>My teacher and I are in the middle of an argument because she says that if you were to drop two objects at the same time and the same height, but with different initial velocities, both of them would hit the ground at the same time. She also said this was proven by Galileo's unregistered experiment when he let two ... | g13877 | [
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<p>When defining tensors as multilinear maps, I am having trouble understanding why a tensor, let's say of type (2,1), can be written in the following way:</p>
<p>$$T = T^{\mu\nu}_{\rho} e_\mu \otimes e_\nu \otimes f^\rho$$</p>
<p>where $\{e_\mu\}$ is a basis for the vector space, and $\{f^\mu\}$ is its dual basis.</... | g13878 | [
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<p>I have the following question for homework:</p>
<blockquote>
<p><em>Show that the Hubble constant $H$ is time-independent in a universe in which the only contribution to energy density comes from vacuum energy.</em></p>
</blockquote>
<p>So in this case we have $\rho = \rho_{\Lambda} = $ constant and $\rho_{\Lamb... | g13879 | [
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0.0014786242973059416,
-0.00045218373998068273,
... |
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