question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>I'm looking at amplitude decrease of a seismic pulse as a result of geometrical spreading.</p>
<p>Starting with the energy contained in a unit area, $I = E / (4 \pi r^2)$, where $E$ is the original energy from source, we know that energy falls off as $1/r^2$, thus amplitude falls off as $1/r$.</p>
<p>From Wikipedi... | g11763 | [
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<p>It is known that good DI water have resistance ~20 MΩ·cm
But how can I measure that? Using good vanilla ohmmeter (with 2000 MΩ range) showed crazy results (too low, not much dependent from distance between probes).
I need that to compare DI water from 2 sources.</p> | g11764 | [
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<p><img src="http://i.stack.imgur.com/TgQNG.jpg" alt="enter image description here"></p>
<p>The picture is an emission spectrum of Helium. The spectrum has sharp lines (peaks) at certain wave lengths characterizing it as helium. Agreed that it characterizes Helium as atomic spectral line characterize gases and they ar... | g11765 | [
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<p>Why does it take a lot of energy to heat metals? And to do it quicker it requires much more energy? What about the use of lasers would that be an efficient process that is quicker and requires less? </p> | g11766 | [
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<p><img src="http://i.stack.imgur.com/H2nRB.jpg" alt="enter image description here"> </p>
<p>Is it possible for the curled dimensions described in superstring theories to become uncurled and open up. I have read that the big bang could have been the uncurling over 3 dimensions through collisions of branes: <a href="h... | g11767 | [
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<p>The wave is</p>
<p>$\bar{E} = E_{0} sin(\frac{2\pi z}{\lambda} + wt) \bar{i} + E_{0} cos(\frac{2 \pi z}{\lambda}+wt) \bar{j}$</p>
<p>Let's simplify with $z = 1$. Now the xy-axis is defined by parametrization $(sin(\frac{2\pi }{\lambda}+wt), cos(\frac{2\pi }{\lambda} + wt)$ where $t$ is time and $\lambda$ is wavele... | g11768 | [
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<p><a href="http://physics.stackexchange.com/questions/38837/spaghettification-of-humans-near-black-holes">THIS POST</a> does a pretty good job of giving an idea as to what the differences in forces becomes as you approach a black hole.</p>
<p>For a small enough black hole, you would reach the dangerous tidal force di... | g11769 | [
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<p>In Hamiltonian mechanics, we show $\{H,L_z\}=0$, which can be interpreted as the conservation of angular momentum around $Oz$. Following the same idea, how can we interprete $\{H,L^2\}$?</p>
<p>Is the interpretation the same (or <em>only</em> similar) as in quantum mechanics for $[H,L_z]$ and $[H,L^2]$?</p> | g11770 | [
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<p>Figure 1.(c) shows the Test image reconstructed from MAGNITUDE spectrum only. We can say that the intensity values of LOW frequency pixels are comparatively more than HIGH frequency pixels.</p>
<p>$$
f(x,y)= ∑_(u=0)^(U-1)∑(v=0)^(V-1)\ |F(u,v)| exp^(1j*2Π*xu)/M) * e^(1j*2Π*(vy)/N) /
$$</p>
<p>Figure 1.(d) shows th... | g11771 | [
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<p>In the book <em>Quantum Mechanics - Volume 1</em> written by Albert Messiah, page no. 142-143, author says: </p>
<blockquote>
<p><em>...But the diaphragm is a quantum object, just like the electron. Its momentum is not defined to better than dp....</em></p>
</blockquote>
<p>I did not understand why diaphragm is ... | g11772 | [
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<p>At what pressure will be particles in a medium be unable to form a sound wave when disturbed? How can this pressure be described mathematically?</p>
<p>My guess is that this would correspond to the point at which the restoring force due to pressure is unable to create a transverse wave and the disturbed particles t... | g11773 | [
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<p>In the theory of non-abelian <a href="http://www.quantiki.org/wiki/Anyons" rel="nofollow">anyons</a>, essential information is stored in the fusion multiplicities or Verlinde coefficients $N_{ab}^c$.</p>
<p>Having the <a href="http://www.math.brown.edu/~brock/home/text/papers/pdwp/www/pdwp.pdf" rel="nofollow">Pants... | g11774 | [
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<p>I'm a novice to physics, so maybe it's rather stupid.</p>
<p>According to <a href="http://en.wikipedia.org/wiki/Tractrix#Basis_of_the_tractrix" rel="nofollow">wiki</a>, the tractrix could be considered a trajectory:</p>
<p>Suppose $AB$ is a stick on a smooth plane $\pi$, and the initial position of $AB$ is $A_0B_0... | g11775 | [
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<p>Can anyone explain why fast neutron reactor designs use sodium/lead/salt cooling, instead of water (heavy/light)?</p>
<p>Is that because neutron absorption by water would not allow to break even in fuel cycle?
Will heavy water help here?</p>
<p>Or water slows neutrons so efficienly so that even if we reduce amount... | g11776 | [
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<p><a href="http://en.wikipedia.org/w/index.php?title=Automatic_quartz&oldid=432281459" rel="nofollow">Wikipedia article on automatic quartz watch</a> describes the watch mechanism as follows: a rotating pendulum is attached to a pinion and when the wearer moves his hand the pinion is rotated at <strong><em>up to 1... | g11777 | [
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<p>Moments after the Big Bang, the universe was expanding at an incredible rate, (I've heard) faster than the speed of light. Due to dark energy, scientists predict the rate of expansion will pick up again. Space itself will be expanding faster than light speed. Someday, we will not be able to see other galaxies becaus... | g11778 | [
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<p><code>5 x 10^7 kg of radioactive material emits energy in the form of photons of red light (700 nm). (Note: photons have no mass.)
What is the momentum of each photon?</code></p>
<p>We just started a new chapter, and in the process of learning it, I came across this question in the back of our book, which I am real... | g11779 | [
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<p>I'm trying to solve the equation:</p>
<p>$$
\frac{\partial^2 v_z(x,y)}{\partial x^2} +
\frac{\partial^2 v_z(x,y)}{\partial y^2} =
\frac{\Delta P}{\mu \Delta X}
$$</p>
<p>This flow is supposed to be flowing in a square channel (laminar,steady,fully developed).</p>
<p>I know I need to solve it numerically and if t... | g11780 | [
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<p>Is there any experiment conducted to prove Many world interpretation of Quantum mechanics?</p>
<p>If it can't be proved why we should take MWI seriously? Recently I read some papers of Max Tegmark who is an advocate of MWI and avoids the lack of experimental prove.</p>
<p>I think <a href="http://physics.stackexcha... | g11781 | [
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<p>For example use p-form for p identical fermion system?</p>
<hr>
<p>The Maxwell equations can be rewritten as a single function using forms plus an intrinsic equation showing the Faraday tensor is a two form. This happens because forms are antisymmetric and we can construct an antisymmetric Faraday tensor which con... | g11782 | [
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<p>The atomic theory as first theorised by Democritus has been successfully applied to matter and to energy (quanta).</p>
<p>Space-time is still generally seen as a continuum. What arguments are there (if any) in support of there being a particulate structure of space-time?</p> | g11783 | [
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<p>As far as I understand Everett's <a href="http://en.wikipedia.org/wiki/Many-worlds_interpretation" rel="nofollow">Many-Worlds Interpretation</a>, he makes the case for a realist theory of QM at the enormous cost of many-worlds.</p>
<p>Are there any arguments in favour of Everett's interpretation that makes the case... | g11784 | [
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<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/3468/what-would-happen-if-you-put-your-hand-in-front-of-the-7-tev-beam-at-lhc">What would happen if you put your hand in front of the 7 TeV beam at LHC?</a> </p>
</blockquote>
<p>Not a terribly scientifi... | g372 | [
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<p>If we were trying to figure out the time scale for a gas-phase reaction between two hydrogen atoms in a molecular cloud (which has density $~10^4/$cm$^3$), apparently the reaction would happen on a time scale proportional to the inverse of the density multiplied by $10^{15}$ years. Aside from the cloud not being den... | g11785 | [
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<p>I once saw a statement about the relation between division algebra(which means you can define a division in this algebra, there is a theorem saying we only have 4 kinds of division algebra, real R, complex C, quaternion H and octonion O) and discrete symmetries, saying that the number of discrete symmetries(I mean t... | g11786 | [
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<p>I am a student studying Mathematics with no prior knowledge of Physics whatsoever except for very simple equations. I would like to ask, due to my experience with Mathematics: </p>
<p>Is there a set of axioms to which it adheres? In Mathematics, we have given sets of axioms, and we build up equations from these set... | g11787 | [
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<p><img src="http://i.stack.imgur.com/B7pBq.png" alt="enter image description here"></p>
<p>I have no idea what formula I'm supposed to use.I just want to know the concept that would allow me to get the answer. I know basic concepts of electricity like charge, currents, etc., but as far resistances go I know only abou... | g11788 | [
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<p>Is it an accurate statement to say that free electrons in a metal experience NO restoring force when they interact with electromagnetic waves? I understand that the electrons exist in a space filled with ions, and doesn't the cumulative potential that is present due to the presence of the ions exert an electric fiel... | g424 | [
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<p><img src="http://i.stack.imgur.com/X9MiJ.png" alt="enter image description here">is it possible to find to resistance without knowing the length?
i am tryigng to solve a problem that requires me to find the resistance.I cant seem do so without knowing the length of the circuit which is not given.Is there something t... | g11789 | [
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<p>I would like to know whether anyone has seen a boundary condition used in a fluid flow problem, of the following type. Suppose viscous incompressible fluid is to the left of a plane $x_1=a$, so the outer normal to the fluid region is $n=(1,0,0)$. The condition specifies that the $x_1$ momentum flux is a nonnegative ... | g11790 | [
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<p>I've seen videos of people in space (on ISS) who squeeze a bottle or something and liquid comes out, it then separates into smaller balls.</p>
<p>Why is this surely it should stay pretty much together because theres no gravity from the Earth so the liquid should attract itself?</p> | g264 | [
0.005319444462656975,
0.03485148772597313,
0.013279203325510025,
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0.07375337183475494,
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0.033303555101156235,
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0.042187873274087906,
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0.034399744123220444,
0.... |
<p>The process is the following:
$$e^-e^+ \rightarrow photon \rightarrow quark + antiquark$$</p>
<p>Regarding the momentum conservation law, how come we have a photon of spin 1 and at the end some meson with spin 0?
Are gluons "behind this"? If this is correct, at which point are they radiated? From quarks? Or? Is th... | g11791 | [
0.007418712601065636,
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<ol>
<li><p>In correspondence to AdS black hole solutions, what does it mean by Misner string singularities? And when there are no Misner string singularities, what does this mean in terms of curvature singularities and event horizons and the black hole in general?</p></li>
<li><p>A related question (as far as I'm awar... | g11792 | [
0.036774035543203354,
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0.022492460906505585,
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0.07415355741977692,
0... |
<p>In <a href="https://en.wikipedia.org/wiki/ENDF" rel="nofollow">ENDF</a> there are cross-sections given for different types of nuclear interactions. For example, <a href="http://t2.lanl.gov/nis/data/endf/endfvii.1-n-pdf/u235.pdf" rel="nofollow">this file</a> gives the cross-sections for different neutron energies. Ho... | g11793 | [
0.0342211052775383,
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0.04005451872944832,
0.012162786908447742,
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0.... |
<p>I'm reading Goldstein's Classical Mechanics and he says the following:</p>
<blockquote>
<p><em>A <a href="http://en.wikipedia.org/wiki/Virtual_displacement" rel="nofollow">virtual (infinitesimal) displacement</a> of a system refers to a change in the configuration of the system as the result of any arbitrary infi... | g11794 | [
0.030398830771446228,
0.013657862320542336,
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0.01893734745681286,
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0.03550814837217331,
-0.0... |
<p>This was a practice test question.</p>
<blockquote>
<p>Consider the following figure. (If the frictional coefficients are unclear, between A and B we have $k_1 = 0.3$, B and platform is $k_2 = 0.2$. If you further doubts about the diagram ask me in the comments below and I will clarify.)</p>
</blockquote>
<p><im... | g11795 | [
0.00901162251830101,
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0.015585627406835556,
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0.00804905965924263,
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0.013... |
<p>I have read that the total magnetic field of a ferromagnet, $\vec{B} = \mu_0\vec{H}+\mu_0\vec{M}$ where $\vec{H}$ is an external magnetic field and $\vec{M}$ is the magnetic field of the ferromagnet due to the alignment of its dipoles to $\vec{H}$. </p>
<p>On the other hand, I have read that ferromagnetic material... | g11796 | [
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0.012... |
<p>In our aerodynamics class we recently discussed the concept of static and dynamic pressure and discussed their application to aircraft instruments. However, I do not understand properly how the altimeter can work.</p>
<p>First of all a small recap of Bernoulli's law. The total pressure is given by $p_t = \frac{1}{2... | g11797 | [
0.05209873989224434,
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0.01... |
<p>If electrons in an alternating current periodically reverse their direction, do they really flow? Won't they always come back to the same position?</p> | g124 | [
-0.0037038486916571856,
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... |
<p>In mechanical engineering, the torque due to a <a href="http://en.wikipedia.org/wiki/Couple_%28mechanics%29" rel="nofollow">couple</a> is given by $\tau = P\times d$, where $\tau$ is the resulting couple, $P~$ is one of the force vectors in the couple and $d$ is the arm of the couple. A couple is made up of two forc... | g11798 | [
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... |
<p>In working through a rigorous derivation of the compressible Navier-Stokes equations, I find that the momentum flux in the X-direction should be driven not only by the normal pressure gradient $\frac{\partial p}{\partial x}$ and shear stress terms $\frac{\partial(\tau_{yx})}{\partial x}$ and $\frac{\partial(\tau_{zx... | g11799 | [
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-0.04... |
<p>The cosmological constant (dark energy) is often described in terms of empty space having a non 0 energy value and this energy being the source of the accelerated expansion of the universe. If space is expanding, what does this imply for energy conservation? Is this energy conserved by being more thinly distributed ... | g11800 | [
0.059548329561948776,
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0.01618173159658909,
0.0213... |
<p>I understand water boils at different temperatures depending on altitude.</p>
<p>I am seeking to get an illustrative explanation for this, including a diagram if possible.</p> | g11801 | [
0.04261390119791031,
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0.05... |
<blockquote>
<p><em>The small, hot, dense early universe the size of an atom was made up entirely of energy, it wasn't until after the expansion began and the universe cooled down some of that energy began converting into the first atomic nuclei.</em></p>
</blockquote>
<p>This quote seems a little dubious but I thin... | g67 | [
0.06481523811817169,
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0.03433195874094963,
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<p>As I understand, in physics, 'information' is closely tied to thermodynamic entropy. Does this relationship imply that if the Universe expands and ends in 'heat death' (maximum entropy?) that it reaches a state of maximum information (so at the Big Bang it had very low entropy and information content)? I sense thi... | g11802 | [
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... |
<p>My lecture script of Quantum Field Theory states that " the state space contains states with negative norm ".</p>
<p>Why does it have to be like this and what would be an example fo such a state?</p> | g11803 | [
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0... |
<p>According to quantum theory,Einstein said that light waves have small packets called photons and they are said to have no mass and charge but having energy E=hv and they behave has both particle and wave character.</p>
<p>And they travel with certain velocity.</p>
<p>How photons have momentum and energy if they sa... | g195 | [
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0.0240... |
<p>On a test, we had a question where there are 4 point charges at the vertices of a square. The 2 charges at the upper vertices have charges of <code>+q</code> and the 2 charges at the lower vertices have charges of <code>-q</code>. The magnitude of the charges are equal. According to the answer sheet, the electric po... | g11804 | [
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0.06512602418661118,
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0.02... |
<p>So I have a triangle Wavefunction defined as:</p>
<p>$$\psi(x)=\begin{cases}x &0<x<\frac{L}{2} \\ L-x &\frac{L}{2}<x<L\end{cases}$$</p>
<p>When I try to find the uncertainty in momentum, I find that it is zero. This cannot be possible by Heisenberg. So I am wondering if I have to (or how to) ... | g11805 | [
0.02233695052564144,
0.015428608283400536,
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... |
<p>It is known the fact that there is no way to extract energy (in any form) from any system without introducing some energy. The Earth for example, gets energy from the Sun, from nuclear fusion of hydrogen. But, what if the Earth would be isolated (there is neither an energy gain, neither loss), it would be possible f... | g11806 | [
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0.022644303739070892,
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... |
<p>Does the knowledge of the material absorption coefficients aids engineers in determining which material to use in their solar cell designs? If yes, how?</p> | g11807 | [
0.04581306502223015,
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0.00105375... |
<p>What is <a href="http://www.encyclopediaofmath.org/index.php/Elliptic_genera" rel="nofollow">elliptic genera</a> in physics?
Reading many relevant papers, they just defined <a href="http://ncatlab.org/nlab/show/elliptic+genus" rel="nofollow">elliptic genus</a> as sort of partition function.
I try to find useful ma... | g11808 | [
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0.030561741441488266,
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-0.02... |
<p>In a video (<a href="https://www.youtube.com/watch?v=KfyWWv5GCzM">Here</a>), I saw crocodiles jump vertically about three meters without using any solid surface. The wonderful thing is that when they start to jump, their vertical velocity is approximately zero, unlike fish who jump using initial velocity. It seems t... | g11809 | [
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<p>If we write the Klein-Gordon equation in this form
\begin{equation*}
c^2 \hbar^2 \nabla^2 \Psi = \hbar^2 \ddot{\Psi} + 2i\hbar (U - mc^2) \dot{\Psi} + U (2mc^2 - U) \Psi
\end{equation*}
we have a pleasant sense of continuity from the non-relativistic to the relativistic treatment of quantum particle (we use the Sc... | g11810 | [
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0.... |
<p>firstly I am a Grade 12 Physics student currently doing my final experimental investigation which I decided to do on double pendulums. Before I got into this, I was under the impression that there was some equation I could use which I could plug in my starting conditions such as starting angle, weight of pendulum ar... | g11811 | [
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<p>I get confused about two things when deriving the time-dependent <a href="http://en.wikipedia.org/wiki/Perturbation_theory_%28quantum_mechanics%29" rel="nofollow">perturbative</a> approach. </p>
<p>We have the Hamiltonian<br>
$$H = H_0 + \lambda H^{(1)}$$
and we have solved (from Schroedinger)<br>
$$\dot{C_f(t)}... | g11812 | [
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0.... |
<p><img src="http://i.stack.imgur.com/4Zvo3.jpg" alt="Question"></p>
<p>My question is above. Firstly, I don't actually know whether it is true or not (!). Secondly, if I were to try to prove it, then I have very little idea how to. The potential steps that I have always done are steps from a constant level to another... | g11813 | [
0.039292868226766586,
0.007704260293394327,
0.020524021238088608,
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<p>I have some questions about the $i\epsilon$ factor in <a href="http://en.wikipedia.org/wiki/Feynman_diagram" rel="nofollow">Feynman diagrams</a>. First, what is the physical meaning of $i\epsilon$ in loop amplitudes. Second, how does it ensures unitarity?
And third, <a href="http://en.wikipedia.org/wiki/Dyson_series... | g11814 | [
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0.06003720685839653,
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0.005... |
<p>A and B are connected to a rope. A constant upward force 86.0N is applied to box A. Starting from rest, box B descends 12.1m in 4.70s . The tension in the rope connecting the two boxes is 32.0N.</p>
<p>What is the mass of B?
What is the mass of A?</p>
<p><img src="http://i.stack.imgur.com/g90hu.jpg" alt="enter ima... | g11815 | [
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-0.0012950398959219456,
-0.002935145748779177... |
<p>Can we divide two vector quantities? For eg., Pressure( a scalar) equals force (a vector) divided by area (a vector).</p> | g11816 | [
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<p>I live in Louisiana these days, in an area that is known for its numerous antebellum plantation homes (circa early 1800s). While touring one of these homes it was clear that almost everything about the house was designed around keeping cool in the summer. Some examples:</p>
<ul>
<li>4 meter high ceilings to allow... | g11817 | [
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0.023583531379699707,
0.0070340014062821865,
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0.08553439378738403,
... |
<p>Would an airplane flying through superfluid helium experience lift and drag? The airplane is presumed cold enough to not heat up the helium.</p> | g11818 | [
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<p>When light rays reflect off a boundary between two materials with different indices of refraction, a lot of the sources I've seen (recently) don't discuss the relation between the amplitude (or equivalently, intensity) of the transmitted/reflected rays and the original ray. Mostly they just discuss the phase differe... | g11819 | [
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<p>My question is basically what exactly is electricity? I've simply been told before that it's a flow of electrons, but this seems too basic and doesn't show that electricity is instant. What I mean is turning a switch has no delay between that and a light coming on. Is it really instantaneous? Or is it just so fast ... | g988 | [
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<p>Why turning velocity of star towards point "A" makes it's satelites, change their apoapsis to side opposite of velocity vector?</p>
<p>Sentence above turned a bit crazy or not understandable (possible reasons: my vocabulary isn't that good; I don't know what the heck I am talking about).</p>
<p>So I mean this: <a ... | g11820 | [
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<p>I'm trying to find the period of precession for a gyroscope. Now I was able to find the angular precession rate, which was 1.132 rad/s, but I have no idea how to convert this to a 'period', and google didnt find me anything useful. How do I do this?</p> | g11821 | [
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<p>Let's say you have a garden hose connected to an ordinary water tap which is opened fully. If you pinch the end of the hose, water leaves the hose at a higher speed (and this can be useful while watering plants, to reach pots which are further away). However when a tap (with no hose connected) is opened only slightl... | g913 | [
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<p>I'm having trouble with a boundary condition. In a fluid mechanics problem, I have flow at $z = \infty$ flowing into a solid plate at $z = 0$ and then flowing radially, and the problem is given as axisymmetric. I know that the velocity field has $v_r$ and $v_z$, and $v_\theta = 0$. How in the world would I expres... | g11822 | [
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<p>Related: <a href="http://physics.stackexchange.com/questions/92960/why-is-stainless-steel-a-poor-conductor-of-electricity">Why is stainless steel a poor conductor of electricity?</a>
I recently had a metal implant, and I'm psyching myself out- should I be considered at all with eddy currents in the implant? I feel l... | g11823 | [
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<p>Help, I am terribly confused about entropy. On the one hand, I am taught at school that a substance such is an ice/solid has a lower entropy than its gaseous equivalent and that a process such as solidification reduces entropy, which is justified with the explanatory premise that the solid is more "ordered". </p>
<... | g11824 | [
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<p>When graphing the induced current in a coil while a magnet is dropped through it why is the total area equal to 0?
The area represents the charge in the coil but why must the resultant flow of charge be 0?</p> | g11825 | [
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<p>For a charge moving in an electric field $\vec E$, its equation of motion is given by the electric part of the Lorentz force $$\frac d {dt}\gamma m \vec v = e\vec E$$This comes from the conservation of relativistic energy in a static electric field. But a magnetic field would still make this conservation law true si... | g346 | [
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<p>A terrifying idea I think, to be so utterly alone.</p>
<p>Do you recon life could exist in intergalactic space? A lot of cosmic radiation is shielded from us by the Milky Way's magnetic field, but the stars of galaxies also create a lot of radiation, so maybe it would cancel out for a planetary system floating in ... | g11826 | [
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<p>I'm interested in the existence of a Lagrangian field theory description of Bronwnian motion, does such a thing exist? Given a particle of some spin $\sigma$, which has a Lagrangian associated with it $\mathcal{L}_{\sigma}$ (which, using the Euler-Lagrange equations, produces Klein-Gordon for $\sigma$ = 0 etc) is t... | g11827 | [
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<p>I been reading: <a href="http://www.mth.uct.ac.za/omei/gr/chap5/node2.html" rel="nofollow">http://www.mth.uct.ac.za/omei/gr/chap5/node2.html</a>. The website seems credible with its contact details & location being extremely accurate and easily verified, and its mathematics seem correct so I have no doubt about ... | g11828 | [
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<p>We know there are rouge stars floating in intergalactic space, thought to be caused by galactic collision. What other other classes of celestial object could be found floating around in intergalactic space?</p>
<p>Within galaxies there are the following types of object:</p>
<ul>
<li>Nebulae </li>
<li>Stars</li>
<l... | g11829 | [
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<p>For a planet which has a temperature gradient, hot in the center and cooler on the surface, why do we see absorption lines?</p>
<p>Similarly, why do we see emission lines if the planet is hot on the surface and gets cooler as you move to the center?</p>
<p>Note, for this question I am only thinking of the planet ... | g11830 | [
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<p>I have a goal to educate myself up to the current level of knowledge we possess about the universe.</p>
<p>I've tried textbooks, wikipedia, lectures, but i find each of them fundamentally flawed in different ways.</p>
<p>Textbooks tend to be incredibly bloated, and I become unable to "see the forest for the trees"... | g98 | [
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<p>I have a Celestron Firstscope telescope and like it overall for my location and the amount of observing I do. I am disappointed in my view of the planets with the scope. What would be a good eye piece to purchase for this telescope to improve planetary observing. In keeping with my limited budget, it should not cost... | g11831 | [
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<p>I understand what an <a href="http://en.wikipedia.org/wiki/Hanbury_Brown_and_Twiss_effect" rel="nofollow">Hanbury Brown and Twiss</a> (HBT) interferometer <em>does</em>, but how can this be used to measure the apparent angular diameter of some object? </p>
<p>What is the mathematical explaination?</p> | g11832 | [
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<p>Could you recommend some good books on physics? I found out that some books on physics are not good, especially about Einstein's Relativity and quantum mechanics. Here is an example which makes me think so: <a href="http://physics.stackexchange.com/questions/110207/while-space-man-lives-for-1-day-then-how-long-does-... | g98 | [
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<p>Is there a detailed description for a <a href="http://en.wikipedia.org/wiki/Hartmann_mask" rel="nofollow">Hartmann mask</a> based collimation process?</p>
<p>I've been told by a friend that is possible to collimate an <a href="http://en.wikipedia.org/wiki/Schmidt%E2%80%93Cassegrain_telescope" rel="nofollow">SCT</a>... | g11833 | [
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<p>We have seen birds sitting on uninsulated electric wires of high voltage transmission lines overhead without getting harmed, because sitting on only one wire doesn't complete any circuit. </p>
<p>But what about the potential difference between their legs? Is this not a small complete circuit? Because the wire has a... | g11834 | [
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<p>A reservoir contains a fluid, and there is an aperture at the bottom through which the fluid flows out as shown in the illustration:</p>
<p><img src="http://i.stack.imgur.com/Ne3dy.gif" alt="enter image description here"></p>
<p>I understand that the rate of flow is based on at least these parameters: the amount o... | g11835 | [
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<p>Problem:</p>
<blockquote>
<p>$1.0 \text{ kg}$ of air at pressure $10^6 \text{ Pa}$ and temperature $398 \text{ K}$ expands to a five times greater volume. The expansion occurs such that in every instance the added heat is a quarter of the work done by the gas. Calculate the final pressure. $1 \text{ kmol}$ has a ... | g11836 | [
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<p>If faster than light travel is impossible, how is it that light emitted from matter so close together in the time soon after the Big Bang is only now just reaching us? I would assume that there would be a "limit" to how far back we can see, but exactly how long after the Big Bang are we able to observe? I'm sure the... | g11837 | [
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<p>I've been working on some exercises and I'm in doubt if my procedure with this one is correct. We have a hollow cylinder with internal radius $r_a$, external radius $r_b$, resistivity $\rho$ and length $L$. A difference of potential $V$ is then applied on the extremes paralel to the axis of the cylinder.</p>
<p>Fir... | g11838 | [
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<p>I am asked to show that an new defined operator:</p>
<p>$$U_{\beta} = \exp(\displaystyle\frac{i\beta L_z}{\hbar})$$</p>
<p>is unitary, where $$L_z = -i\hbar\,\,(x\displaystyle\frac{\partial}{\partial y} - y \frac{\partial}{\partial x }).$$</p>
<p>I tried the following: </p>
<p>$$ U_{\beta}^{\dagger} U_{\beta} = ... | g11839 | [
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<p>For example, there's a very simple circuit which only contains on resistor. So according to Ohm's law, we have: $\mathrm{emf} = IR$</p>
<p>As we know when time $t = 0$, the current must be $I = 0$. However, how do I describe how the current really behaves just after I switched on the circuit?</p>
<p>You may ask wh... | g11840 | [
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<p>My understanding is that the Casimir Effect is caused by vacuum energy. Quantum mechanics (QED) predicts vacuum energy, but gets the value grossly wrong, by a factor of $10^{120}$. On the other hand, from what I have read, Dark Energy is understood to be caused by vacuum energy.</p>
<p>Has anyone checked whether th... | g11841 | [
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<p>I'm writing a script involving physics equations, and someone complained that my script outputs $F = m a$ as $F = a m$, as well as outputting $E_p = m g h$ as $E_p = g h m$; another example would be $E = m c^2$ vs $E = c^2 m$. I've obviously opted for displaying the variables in the equations in alphabetical order, ... | g11842 | [
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<p>I'm tough in school that a reference frame must be non-moving; For example if I take as reference frame the waves of the ocean, i will have the impression that i'm moving, but I'm not. But if movement is relative, how to distinguish between a reference frame that is not moving and one that is moving? It is impossibl... | g11843 | [
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<p>Is it reasonable to assume that de Sitter temperature will stop growing when it reaches equilibrium with the temperature of CMB?</p>
<p>Currently the de Sitter temperature (the temperature of the Universe's boundary) is $$T_{dS}=2.67\times10^{-30}\,\mathrm K$$</p>
<p>and rises.</p>
<p>At the same time, the temper... | g11844 | [
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<p><em>"The electron around the nucleus is in a quantized energy level and can change it only if an external interaction intervenes."</em> That is OK but when there is a magnet, it has energy of attracting iron particles for months and years (natural magnet) just by right order of atoms. Where this energy come from? I... | g202 | [
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<p>When a body lies on the surface of the Earth it is under the influence of gravity. The force on the body due to gravity causes it to exert a force on the ground and the normal reaction acts in the opposite direction causing the resultant force on the body to be zero.</p>
<p>However, how can the body exert a force o... | g63 | [
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<p>I just wanted to know whether time dilation is different from time travel or not. Personally I believe both are different. If they are the same things I just wanted to know why. According to me time dilation is something that applies for every object, while time travel particularly applies for humans. Moreover I fee... | g11845 | [
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<p>I just wanted to understand the following. Let's stay with the harmonic oscillator in QM, just to have an example at hand.
First, there are all the different states for $n=1,2,...$. (Let's call them $\psi_n$).</p>
<p>Then, the superposition of a state, for example $$\frac{1}{\sqrt{2}}(\psi_1+ \psi_2)$$ is also a s... | g11846 | [
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<p>From a physicist's perspective there are several situations in which somehow arbitrary choices of mathematical structures can be made. One can describe a system from different perspectives, etc. without changing anything in the physical properties. (see gauge invariance, picture changing etc.)
In the situation when ... | g373 | [
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<p>Is there any physical significance of the fact that the group manifold (parameter space) of $SO(3)$ is doubly connected?</p>
<p>EDIT 1: Let me clarify my question. It was too vague. There exists two equivalence classes of paths in the group manifold of SO(3) or in other words, $\Pi_1(SO(3))=Z_2$. This space is ther... | g11847 | [
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<p>I think I am currently making a mistake regarding my interpretation of Faraday's Law.</p>
<p>$\vec{\nabla}\times\vec{E}=-\frac{\partial\vec{B}}{\partial t}$</p>
<p>Assuming we have an electric field $\vec{E}$ in the plane between 2 particles given by:</p>
<p>$\vec{E}=(\frac{x}{x^2+y^2}+\frac{1-x}{(x-1)^2+y^2}, \f... | g11848 | [
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