question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>In Newton's model of light as being composed of particles, it's easy to imagine reflection as being the rebounding of individual corpuscles off a surface. However, since light can also behave like a wave, it poses a challenge in visualizing reflection.</p>
<p>How does a wave reflect off of a surface, whether it be ... | g92 | [
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<p>So, I've just been reading a recent <a href="http://www.sciencedaily.com/releases/2011/09/110914171756.htm" rel="nofollow">ScienceDaily</a> article, and came across this interesting quote</p>
<blockquote>
<p>"Clays form in more neutral, less acidic conditions than the
sulfate-rich sandstones we've been looking ... | g18212 | [
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<p>I frequently hear that Kepler, using his equations of orbital motion, could predict the orbits of all the planets to a high degree of accuracy -- <em>except Mercury</em>. I've heard that mercury's motion couldn't be properly predicted until general relativity came around. But what does <a href="http://en.wikipedia.o... | g587 | [
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<p>I am currently doing the following problem:</p>
<blockquote>
<p>Two identical undamped oscillators, A and B, each of mass m and natural (angular) frequency $\omega_0$, are coupled in such a way that the coupling force exerted on B is
$$\alpha m(\frac{d^2 x_A}{dt^2}), $$</p>
<p>and the coupling force exerte... | g18213 | [
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<p>Link to the original paper: <a href="http://arxiv.org/abs/hep-th/0312070" rel="nofollow">The Gauge/String Correspondence Towards Realistic Gauge Theories (arXiv paper)</a></p>
<p>On page 52 we see that, for a theory of Dp-branes placed at an orbifold (orbifold = $C_{2}$/$Z2$) fixed point, we have a certain spectrum... | g18214 | [
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<p>My teacher said we only consider Lagrangians which are quadratic in $\dot{q}$, and we don't take other Lagrangians. I couldn't understand why. Can anyone please explain this?</p> | g564 | [
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<p>Looking at the equation for binding energy and mass defect,<br>
$$
B = m_{\text{free}} - m_{\text{bound}} \\
\Rightarrow m_{\text{bound}} = m_{\text{free}} - B,
$$
my question is the following. Suppose that the binding energy $B$ becomes larger than the free rest mass $m_{\text{free}}$. It would seem that the mass o... | g18215 | [
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<p>How much energy/work will it take if our planet is:</p>
<pre><code>5.9736×1024 kg
</code></pre>
<p>or </p>
<pre><code>3.0×10−6 Suns
</code></pre>
<p>to move Sun and Earth 1 meter closer to each other?</p> | g18216 | [
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<p>Every planet and satellite revolves around their mother planet and about their own axis as we know. So they are compacted as a system. Then why not sun?</p> | g18217 | [
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<p>What does the height refer to in the <a href="http://en.wikipedia.org/wiki/Bernoulli%27s_equation" rel="nofollow">Bernoulli equation</a>?</p>
<p>I'm trying to determine the pressure on a surface where the length is short enough that the velocity should be constant across it.</p>
<pre>
Fan -> -8_________________
... | g18218 | [
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<p>Yes, a <a href="http://en.wikipedia.org/wiki/Rainbow" rel="nofollow">rainbow</a> is formed when a raindrop refracts light, but why then does the whole sky not become a huge rainbow when it rains? Would the light not be dispersed into ordinary 'white' light? What forces it to look as if each end is nearly touching th... | g18219 | [
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<p>I am looking for the commutator:
$$[e^{aq},p]$$
My approach is to Taylor expand the function:
$$[\sum_n \frac{1}{n!}(aq)^n,p]$$
I know that $[q^n,p]=ni\hbar q^{n-1}$
So how do I account for $n$ commutators?</p> | g18220 | [
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<p>Before <a href="http://enwp.org/imaginary_unit" rel="nofollow">I</a> <a href="http://physics.stackexchange.com/questions/75436">write</a> <a href="http://physics.stackexchange.com/questions/75436">this question</a>, I'd want to say that I've read <a href="http://physics.stackexchange.com/questions/46983/... | g18221 | [
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<p>I had read that light does slow down in glass because photons interact with atoms in glass. They are absorbed and re-emitted and during this phenomenon it's speed decreases. See also <a href="http://physics.stackexchange.com/q/466/2451">this</a> and <a href="http://physics.stackexchange.com/q/11820/2451">this</a> P... | g18222 | [
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<p>When the spherical side of a planoconvex lens is towards the source we will see reduced aberrations normally we use it for the beam collimation. When we use two planoconvex lenses configuration why we dont use back to back oriented like a single planoconvex? why do we use them as in the second picture below. What wi... | g18223 | [
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<p>I am interested to measure the thickness of the coating on a mirror (e.g. in the picture below). What are the available and best way of measuring it? What will be the solution in case of multiple layer coating on the same mirror how we could measure the thickness of all layers?</p>
<p><img src="http://i.stack.imgur... | g18224 | [
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<p>In quantum mechanics, we usually write the Hamiltonian as:</p>
<p>$$\hat{H}=\hat{T}+\hat{V}$$</p>
<p>But in classical mechanics, there are several reasons why it <strong>would not have this form</strong>:</p>
<ul>
<li>We've chosen some particular coordinate system where there is no conservation of energy (we're n... | g18225 | [
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<p>Imagine the following:</p>
<pre><code>A-> [] <-B
</code></pre>
<p>where [] is a black hole, and A and B are two objects of same mass, traveling with the same speed right at the black hole.</p>
<p>A and B have immense kinetic energy. When they crash into the black hole, the black hole does not feel any net... | g18226 | [
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<p>It is said that <em>the law of least action is that nature tries to convert potential energy into kinetic one as fast as possible.</em></p>
<p>Information can't be thought without a physical realisation, see <a href="http://philosophy.stackexchange.com/q/9243/1127">here</a>. It may be thought as a form of entropy. ... | g18227 | [
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<p>In my Calculus class, my math teacher said that differentials such as $dx$ are not numbers, and should not be treated as such.</p>
<p>In my physics class, it seems like we treat differentials exactly like numbers, and my physics teacher even said that they are in essence very small numbers. </p>
<p>Can someone giv... | g18228 | [
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<p>What happens when a proton collide with an atom? How does the ionization occurs, assuming the proton has enough kinetic energy? How much energy is required for the proton to enter the atomic nucleus, resulting a higher chemical element ion?</p> | g18229 | [
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<p>Let us assume that we want to describe the full process of photon emission by electron A and absorption by electron B.</p>
<p>Therefore electron B must be on the forward lightcone of electron A.</p>
<p>In the normal forwards in time description a virtual photon propagates from A to B depositing a certain amount of... | g18230 | [
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<p>I'm trying to solve the inflation problem for exponential potential.
$$v(\phi) = v_0 \exp(-\alpha \phi)$$
(it's known as barrow or power law inflation)
we have two main equations:
$$H^2 = 8π G / 3 (1/2 (\dot{\phi})^2 + v(\phi))$$
$$\ddot{\phi} + 3H \dot{\phi} + v(\phi)'=0$$
I must solve these two equations and find ... | g18231 | [
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<p>I don't know much about <a href="http://ncatlab.org/nlab/show/tangent+bundle" rel="nofollow"><i>tangent spaces</i>, or <i>tangent vectors</i>, "as such"</a>; nor about <a href="http://en.wikipedia.org/wiki/Schild%27s_ladder#affine_parametrization" rel="nofollow"><i>affine parametrization</i></a> (which seems to be c... | g18232 | [
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<p>I'm curious, are there specific conditions for a transfer of mass between two stars as they pass near each other? Its for my final project in computing, I'm writing a gravity simulation and thought it'd be a nice touch.</p> | g18233 | [
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<p>I recently saw a video, <a href="http://youtu.be/ZMByI4s-D-Y" rel="nofollow">World's Roundest Object</a>, which discussed a sphere made of silicon-28. The researcher in the video stated, if the object was scaled to the size of the Earth, the highest and lowest points would only differ by 14 meters in height.</p>
<p... | g18234 | [
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... |
<p>I read a proof of Liouville’s theorem on integrable Hamiltonian systems. According to the theorem, an autonomous Hamiltonian system can be integrated in quadratures, given $n$ involutive first integrals $F_j(q_1\dots q_n,p_1\dots p_n)=a_j,\quad j=1\dots n$, if the Jacobian $\frac{D(F_1,\dots,F_n)}{D(p_1,\dots,p_n)}$... | g18235 | [
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<p>I am not an expert of string theory and am quite uncertain about the basic ideas of string theory that I am going to ask about. I would appreciate some hints of more experienced physicists.</p>
<p>What I am trying to understand is how string theory can describe particles as quantum fields as in quantum field theory... | g18236 | [
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<p>A solid cuboid has the following moment of inertia:</p>
<p>$$I= \begin{pmatrix}
\frac{1}{12}h^2+d^2 & 0 & 0 \\
0 & \frac{1}{12}w^2+d^2 & 0 \\
0 & 0 & \frac{1}{12}w^2+h^2
\end{pmatrix} $$</p>
<p>When integrating the torque to the angular velocity, as far as I know, this holds:</p>
<p>... | g18237 | [
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<p>Apologies in advance, I'm a layman with only a school-level education in physics.</p>
<p>If an object approaching the event horizon of a black hole has its light cone progressively bent towards the black hole's singularity, why can all observers in the future not see the light bouncing off of it?</p>
<p>In a recen... | g18238 | [
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<p>So recently one of my professors went off on a tangent, and we ended up discussing atomic clocks and how they work, which is something I've always been fascinated with and thoroughly enjoyed. But it left me with a question he couldn't answer. </p>
<p>After you've built an atomic clock (or any clock for that matte... | g18239 | [
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<p>I am working on a project that requires solar panel to rotate around its axis, but I am struck with deciding the specs of the Servo Motor that is capable of doing this. Though, the panel hangs on its axis with help of two axle rods on each side attached almost in the mid of the length, and the servo is to be attache... | g18240 | [
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<p>I have some experimental data about a value $n$, now, I am supposed to give, in the ending, a single value with an error: $n=a\pm b $. I have originally 6 values of $n$, each one comes as an indirect measurement from direct measurement, each one with it's systematic errors, so in the ending I have those 6 values, ea... | g18241 | [
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<p>I'm trying to model a system with two masses, two springs, two dampers, and one applied force using transfer functions. I'll then be inputting it into Simulink.</p>
<p>The system looks like <a href="http://upload.wikimedia.org/wikipedia/commons/f/fd/Mass-Spring-Damper_%282_body_system%29.svg" rel="nofollow">this</a... | g18242 | [
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<p>This is a question about probability. The <a href="http://en.wikipedia.org/wiki/Bean_machine">Galton box (or quincunx)</a> uses the physical process of shot moving down a pin-board, to demonstrate central limit theorem, eg:</p>
<p><img src="http://i.stack.imgur.com/OQQXg.png" alt="quincunx"></p>
<p>So I am interes... | g18243 | [
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<p>Why are the energy levels of molecules, the <a href="http://en.wikipedia.org/wiki/Atomic_physics" rel="nofollow">atoms</a> that form them and the <a href="http://en.wikipedia.org/wiki/Nuclear_physics" rel="nofollow">nuclei</a> inside the atoms considered separately? Or phrased in a different way- what is it that mak... | g18244 | [
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<p>Do the "operator for translations in superspace" and the "position in superspace operator" follow an uncertainty principle? How "real" is <a href="http://en.wikipedia.org/wiki/Superspace" rel="nofollow">superspace</a>? Aside from being weird (and possibly just a "mathematical trick") is superspace really just an e... | g18245 | [
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<p>In Quantum Mechanics the value of an observable results from the interaction between the "system" with the "Measuring gadget".</p>
<p>But when the experimenter[or the technologist concerned] is devising/constructing the gadget he has a view the "system" with its stand alone properties/attributes.It seems to be so.<... | g18246 | [
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<p>Does 't Hooft's determinism work need the holographic principle in order to work or is it just an extension of his work?</p> | g18247 | [
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<p>I thought of this question; would too much wiring make a computer burn down? Or can you build an actual full-speed supercomputer computer using switches and wires?</p>
<p>In other words, could simply adding wires and switches into a computer already composed of switch and wire, can you achieve a calculation speed w... | g18248 | [
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<p>How would one calculate the gravity vector for [X,Y,Z] by using an inclinometer which gives Pitch, Yaw and Roll.</p>
<p>A little explanation of the goal: I have a accelerometer I was planning to use to map movement. But as gravity being what it is, it affects everything.</p>
<p>So the Accelerometer reports [aX,aY,... | g18249 | [
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<p>Torque is just the turning force on an object, i.e the force multiplied by the perpendicular distance from the force to the pivot.</p>
<p>But when <a href="http://www.google.com/search?as_epq=torque+parameters" rel="nofollow">torque parameters</a> are mentioned in mechanical engineering papers, what are they referr... | g18250 | [
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<blockquote>
<p><em>Suppose we have a disk of radius $r$ and mass $m$ travelling at velocity $v$. I want to calculate the instantaneous angular momentum with axis through the edge of the disc (on the circumference).</em></p>
</blockquote>
<p>Angular momentum $= I \omega$. $I = \frac{1}{2}mr^2 + mr^2 = \frac{3}{2}mr^... | g55 | [
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<p>Here's a quote from Wikipedia:</p>
<blockquote>
<p>As an example, the ground state configuration of the sodium atom is 1s<sup>2</sup>2s<sup>2</sup>2p<sup>6</sup>3s, as deduced from the Aufbau principle (see below). The first excited state is obtained by promoting a 3s electron to the 3p orbital, to obtain the 1s<... | g18251 | [
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<p>Phase transitions in statistical mechanics are usually taught by working through a bunch of examples. I decided to try and think about them from a more "fundamental" point of view - but I've run into a weird little snag, in that I've come up with a system that seems to satisfy the most common definition of a (first ... | g18252 | [
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<p>I understand that the expansion rate of the universe driven dark energy exceeds the rate at which the local group is being pulled toward The Great Attractor (Hydra-Centaurus Supercluster). Does this mean that the local group will be isolated from the Great Attractor and all other galaxies in 40 billion plus years.</... | g18253 | [
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<p>I understand gauge theory as the theory of continuous transformation group which keeps Lagrangian (or dynamics) invariant. So some integral invariants could be found. In terms of classical electromagnetism, we choose gauge so that obtain different vector potentials $A$. My question is, since now different potentials... | g18254 | [
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<p>I read in web that Einstein used the <a href="http://en.wikipedia.org/wiki/Equivalence_principle" rel="nofollow">principle of equivalence</a> to explain General Relativity but we know the gravitation is approximately equal in all of rested frame in gravitional field. In contrary, we can only create uniform accelera... | g18255 | [
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<p>How tight are experimental and astrophysical tests on whether Cluster Decomposition is satisfied at various space-like separations?</p>
<p>Is there a review paper or a standard reference on the question?</p>
<p>I note that Cluster Decomposition may be related to the existence of a unique vacuum state and to <a hre... | g18256 | [
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<p>The photoelectric effect demonstrates that EM radiation has particle-wave duality and it can be used to generate electricity. I am still a novice in physics and my understanding of this phenomenon is that when the energy in photon is high enough, it can remove electrons from an atom, and with higher energy it can in... | g18257 | [
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<p>I've heard that resistors are used to decrease current to a particular appliance, such as in the regulator of a fan. However, I've also heard that the total current in a circuit is always the same- in other words, all the current leaving the positive terminal of the battery reaches as it is to the negative terminal.... | g18258 | [
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<p>Is this a correct definition of electrostatic induction, The <strong>production</strong> of opposite charges on a neutral body when a charged body is brought near to it? I think it's not the production but redistribution.</p> | g18259 | [
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<p>Bipartite operators close enough to the identity are separable. But how does one compute the product operator terms of the separable expansion? In particular, if $\left| \Phi \right> = m^{-1/2} \sum_{i=1}^n \left| i \right> \otimes \left| i \right>$ is the $n$-dimensional Bell state then $I \otimes I - \l... | g18260 | [
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<p>I am reading Feynman and Hibbs on Path Integrals. In section 3.5, they show that the kernel for a lagrangian of the form $L=a(t)\dot{x}^2+b(t)\dot{x}x+c(t)x^2+d(t)\dot{x}+e(t)x+f(t)$ is $K(b,a)=e^{\frac{i}{\hbar}S_{cl}[b,a]}F(t_a,t_b)$. In general, how do I calculate the factor $F(t_a,t_b)$. In the problems after th... | g18261 | [
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<p>Suppose that fast quantum random number generator (QRNG, <a href="https://qrng.physik.hu-berlin.de/" rel="nofollow">https://qrng.physik.hu-berlin.de/</a>) is placed in a subsystem which has limited interaction with
the rest of the world. What would happen if decoherence rate for this subsystem
were not fast enough t... | g18262 | [
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<p>The <a href="http://en.wikipedia.org/wiki/Stick-slip_phenomenon" rel="nofollow">slip-stick phenomenon</a> is present all around us, be it the noise of car breaks or in earthquakes. But does it have any real-life application?</p> | g18263 | [
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<p>If a large star goes supernova, but not enough mass collapses to form a black hole, it often forms a neutron star. My understanding is that this is the densest object that can exist because of the Pauli exclusion principle: It's made entirely of degenerate matter, each particle of which cannot occupy the same quantu... | g208 | [
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<p>Here is my current dilemma: I used the data from <a href="http://irsa.ipac.caltech.edu/Missions/msx.html" rel="nofollow">MSX6 Survey</a> and want to put a citation or some kind of acknowledgment but I can't seem to find any concrete paper or reference text to put in the paper. Any of you would know what paper to cit... | g18264 | [
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<p>Inspired by <a href="http://physics.stackexchange.com/questions/111514/rocky-planet-in-the-center-of-system">this</a> question, which sort of asked what I'm asking, but emphasized a different point (and the answers accordingly addressed that). </p>
<p>Can a rocky planet form directly from the collapse of an interst... | g18265 | [
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<p>Why do wind power plants have just 3 blades? It seems that adding more blades would increase the area that interacts with the wind and gather more energy.</p>
<p><img src="http://i.stack.imgur.com/KOC1w.jpg" alt="enter image description here"></p>
<p><sub>(Image from <a href="https://en.wikipedia.org/wiki/File%3aW... | g565 | [
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<p>shortly after the invention of <a href="http://en.wikipedia.org/wiki/Quantum_electrodynamics">quantum electrodynamics</a>, one discovered that the theory had some very bad properties. It took twenty years to discover that certain <strong>infinities</strong> could be overcome by a process called <a href="http://en.wi... | g18266 | [
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0.03686347231268883,
-0.024796301499009132,
0.0... |
<p>I understand that energy conservation is not a rule in general relativity, but I'd like to know under what circumstances it can still be possible. In other words, when is it possible to associate a potential energy to the gravitational field, so that the energy is constant in the evolution of the system?</p>
<p>Her... | g18267 | [
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<p>I had an interesting thought today that caused me to ask whether it'd be possible to make corrections to the ideal gas law via introducing terms derived from more realistic phenomena to make their ways into the equation. </p> | g18268 | [
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<blockquote>
<p>Consider a body attached to a horizontal spring and resting on a surface, inclined at an angle $\theta$ from the ground.<img src="http://i.stack.imgur.com/F2G97.png" alt="Set-up"></p>
<p>The spring constant is $k$. Initially the spring was kept in its natural length while the body was held still ... | g18269 | [
0.08786169439554214,
0.027056463062763214,
0.008270048536360264,
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0.029242178425192833,
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0.01463... |
<p>What are <a href="http://en.wikipedia.org/wiki/Killing_spinor" rel="nofollow">Killing spinors</a>? How can they be motivated? Are they directly related to <a href="http://en.wikipedia.org/wiki/Killing_vector_field" rel="nofollow">Killing vectors</a> and <a href="http://en.wikipedia.org/wiki/Killing_tensor" rel="nofo... | g18270 | [
-0.017147118225693703,
0.0152510991320014,
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0.021037450060248375,
0.09412209689617157,
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0.06775955110788345,
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0.0031005670316517353,
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0.016048604622483253,
0.07133074849843979,
-0.041... |
<p>In a recent discussion about black holes, space_cadet provided me with the following paper of Rovelli: <a href="http://arxiv.org/abs/gr-qc/9603063">Black Hole Entropy from Loop Quantum Gravity</a> which claims to derive the Bekenstein-Hawking formula for the entropy of the black hole.</p>
<p>Parts of his derivation... | g18271 | [
0.002268834738060832,
0.015703856945037842,
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0.05526223033666611,
0.005372... |
<p>While driving, it takes 22.0 s to pass a car. If my speed was 64.0 mph when I started to pass, and was 80.0 mph as I finished, how far (in feet) did I travel while passing?</p> | g18272 | [
0.01974359154701233,
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<p>Exchange polarization is the process by which spin is transferred between an electron beam and a system of polarized atoms (with a single valence spin). The process occurs as a result of the Pauli Exclusion principle. See Burke and Schey Phys. Rev. 126 163 (1962).</p>
<p>The type of scattering processes that can oc... | g18273 | [
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<p>Based on what is measurable, how much energy is in the universe if the initial time is the big bang and the final time is the present?</p> | g27 | [
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<p>What is a <a href="http://en.wikipedia.org/wiki/Covalent_bond" rel="nofollow">covalent bond</a>, quantum mechanically? How does it hold the two atoms together, and at one point can you qualify the electron as being shated between two atoms, versus feeling attractive forces?</p> | g18274 | [
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<p>We all know that Isaac Newton developed the gravitational theory (as is often told) when an apple fell on his head. </p>
<p>But my question is, didn't anyone before him notice it?</p> | g18275 | [
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0.00639... |
<p>Related:<a href="http://chemistry.stackexchange.com/q/9459/">http://chemistry.stackexchange.com/q/9459/</a></p>
<p>I was wondering, <a href="http://en.wikipedia.org/wiki/Covalent_bond" rel="nofollow">covalent bonds</a> tend to share two electrons, apparently rarely more than three, and normally between two electron... | g18276 | [
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<p>In a non-dispersion medium, phase velocity is equal to group velocity of waves. Consider a free quantum particle moving in one dimension say $\text{x}$ axis. There would be matter waves associated with the particle. </p>
<p>According to derivations available in books, the phase velocity is half the velocity of pa... | g18277 | [
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<p>Imagine a cubic lattice for simplicity and put some atomic configuration inside of each unit cell. Lattice is of course a highly ordered entity (long range order). Please, how could I quantitatively describe this via some correlator function? I assume no up/down spins are present on the lattice.</p>
<p>This is most... | g18278 | [
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<ol>
<li><p>What is the difference between <a href="http://en.wikipedia.org/wiki/Dot_product" rel="nofollow">dot product</a> and <a href="http://en.wikipedia.org/wiki/Cross_product" rel="nofollow">cross product</a>? </p></li>
<li><p>Why do we use cross product to find torque, why can't we use dot product?</p></li>
<li>... | g18279 | [
0.01932227984070778,
0.006319133564829826,
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-0... |
<p>I am only the equivalent of a high school student, so please, if possible, don't answer this question with anything too complex or really advanced university level. I am very happy to research new concepts anyone mentions, but can you please keep it reasonably simple.</p>
<p>With transformer windings, primary vs se... | g18280 | [
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... |
<p>This is related to <a href="http://physics.stackexchange.com/questions/6084/dimensional-reduction-from-31-to-21-for-caln-2-vector-superfield">this earlier question</a> I had asked. </p>
<p>I am using the so called ``Majorana" representation of gamma matrices in $2+1$ dimensions in which everything is real. After do... | g18281 | [
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... |
<p>This question is from Principles of Quantum Mechanics by R. Shankar.</p>
<blockquote>
<p>Given the operator (matrix) $\Omega$ with eigenvalues $e^{i\theta}$ and $e^{-i\theta}$ , I am told to find the corresponding eigenvectors.</p>
</blockquote>
<p>I think these equations are relevant:</p>
<p>$$\begin{align}\Om... | g18282 | [
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0.023... |
<p>To see the full height of man, the height of mirror should be half of the height of the man. What happens when we decrease the distance between mirror and man?</p> | g18283 | [
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<p>Is there any probability that a <a href="http://en.wikipedia.org/wiki/Gamma-ray_burst" rel="nofollow">gamma-ray burst</a> (GRB) could affect the Earth?</p> | g18284 | [
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<p>You throw a baseball directly upward at time t = 0 at an initial speed of 12.1 m/s. What is the maximum height the ball reaches above where it leaves your hand? At what times does the ball pass through half the maximum height? Ignore air resistance and take g = 9.80 m/s2. Find the Earlier time at half maximum height... | g18285 | [
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<p>I got a question concerning the band strucutre of solids.
The reference I'm using is the book on solid state physics by Ashcroft/Mermin.</p>
<p>My problem is that I don't completely understand the reason why there exists a
band-index n. On the one hand Ashcroft gives a quite plausible explanation
saying that the ... | g18286 | [
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<p>I've just finished a class a few weeks ago which taught thermo and stat mech, and I still don't know the exact difference between the two. Can someone help clear this up for me? (Yeah it's sad, and even sadder that I got an A- in the course)</p> | g18287 | [
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<p>And expansion of space is equal to expansion of vacuum?</p> | g18288 | [
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<p>It is known that a superconductor is a material with electrical resistance zero. My question is, it is exactly zero, a theoretical zero, or for practical realistic reasons it is effectively zero?</p> | g18289 | [
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<p>Motivated by the answers to <a href="http://physics.stackexchange.com/questions/20850/occurrence-of-turbulences-in-fluid-dynamics-from-the-equations-of-motion/20868#comment48723_20868">this question</a> on turbulence, I'm interested in an explanation and/or derivation/reference of the <a href="http://en.wikipedia.or... | g18290 | [
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<p>I'm writing down the partition function for a system, for which I know the dispersion relation</p>
<p>$$E \left( \mathbf{k} \right) = \sqrt{ \left| \mathbf{k} \right|^2 + m^2 + \cdots }$$</p>
<p>The exact form is not important, what matters is that technically, as the dispersion relation is the solution of a 2nd d... | g18291 | [
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<p>In standard college physics text books, high-school books and popular level physics books, the collapse of the Tacoma Narrows Bridge is often taken as an example of <em>resonance</em>. However a more detailed analysis shows that the collapse was not due to simple resonance (see the article by <a href="http://www.ket... | g18292 | [
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<p>Let's say an array of rays of light is given.<br>
Each ray has a specific wavelength (in the range of visible light). </p>
<p>Example:</p>
<pre><code>ray1 = 470 nm
ray2 = 640 nm
ray3 = 510 nm
</code></pre>
<p>Can these rays be merged into a single ray of "white light" which preserve the information of it's compo... | g18293 | [
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<p>Assume that by some means, the moon could be given an atmosphere, of the same density and pressure at the surface as the earth's. Obviously in a stable atmosphere there are temperature variations from pole to equator, day to night, and a lot of interesting dynamics follow from that, but for argument's sake assume an... | g18294 | [
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<p>Say I have a qubit with probability amplitude divided evenly among $|0\rangle$ and $|1\rangle$
$$\frac{1}{\sqrt 2}|0\rangle + \frac{1}{\sqrt 2}|1\rangle$$</p>
<p>So it seems that we have a, loosely speaking, 'symmetric' probability amplitude for the states. However I have read that this is not the case and </p>
<p... | g18295 | [
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<p>I mean the nitty-gritty measurements of positions of stars and planets using transit circles. The transit instruments I've seen are mostly in national observatories and are essentially museum pieces: in Greenwich, U.S. Naval Observatory in Washington, Ottawa, Sidney, Paris, etc. I've never read of anybody in the pre... | g18296 | [
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<p>We seem to know that black hole energy, and apparently event horizon geometry and topology, are functions of Mass, Charge and Angular Momentum. Area is a function of geometry and topology as well, which apparently controls entropy. Strominger, et al, can show that the apparent agreement of string states to event h... | g18297 | [
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0.029779452830553055,
0.024744858965277672,
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0.006678522564470768,
0.005429293494671583,
-0.01259977649897337,
0.0543115995824337,
-0.01... |
<p>What I am interested to see are specific examples/reasons why gravity should be quantized. Something more than "well, everything else is, so why not gravity too". For example, isn't it possible that a quantum field theory on curved space-time would be the way treat QFT and gravity in questions where the effects of n... | g18298 | [
0.04680652171373367,
0.04214860498905182,
0.00010331387602491304,
0.0036836382932960987,
0.051224540919065475,
0.010275506414473057,
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-0.026403984054923058,
0.04936101287603378,
-0.013178884983062744,
-0.018581466749310493,
... |
<p>Other physics communities, e.g. the particle physics one, have their own set of units, custom-tailored to their own needs. Now, the astrophysics community is somewhat similar, in that a lot of measurements are often expressed in terms of corresponding numbers related to the Sun (solar mass, solar luminosity, solar r... | g18299 | [
0.009870163165032864,
0.050076838582754135,
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0.0018166896188631654,
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-0.0005802215891890228,
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0.... |
<p>I'm trying to understand tensors, but I've come across the following question:</p>
<blockquote>
<ol>
<li>Let $T^{\mu\nu}$ by a $(2,0)$ tensor. Give the definitions of $T_\mu^{\,\nu}$, $T_{\mu\nu}$, and $T^{\mu}_{\,\nu}$. Then give explicit formulas for the following components in terms of components of $T^{\mu\... | g514 | [
-0.025518866255879402,
0.02700728364288807,
-0.01699863374233246,
-0.010559897869825363,
0.03597453609108925,
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0.014124128967523575,
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0.04114064574241638,
0.07938411086797714,
-0.04... |
<p>In arXiv:0710.4330v1 Balitsky calculate the eikonal scattering of dipole composed of quark anti-quark, $Tr(U_{x}U^{\dagger}_{y})$, to NLO accuracy. The result he found is:
<img src="http://i.stack.imgur.com/EGe1e.png" alt="enter image description here"></p>
<p>Where $\mu$ is the normalization point in the MS bar sc... | g18300 | [
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0.013466579839587212,
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0.0071312119252979755,
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0.016790756955742836,
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-0.0046033659018576145,
0.029177837073802948,
0.008005703799426556,
0.06985297054052353,
... |
<p>I have a Meade LX10 and have never done any astrophotography (except with some computerised SBIG ST9s on my university's LX200s). I would like to get started at home, but could do with some advice on which low budget camera would be suitable, obtainable in the UK and preferably under £100. Do I need a DSLR? I've see... | g18301 | [
0.01625465415418148,
-0.005001130513846874,
0.0005451589240692556,
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0.010238970629870892,
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-0.04593440145254135,
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0.06617950648069382,
0.053751878440380096,
0... |
<p>I read that glass has a <a href="http://en.wikipedia.org/wiki/List_of_thermal_conductivities" rel="nofollow">thermal coefficient of 0.8-1 W/mK</a>. Given that my window window thickness is around $5\ mm$, then I would calculate my heat loss per area being $160-200\ Wm^{-2}K^{-1}$. </p>
<p>Yet standard glass has a U... | g18302 | [
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0... |
<p>In section 5 of the "Preliminary: On the measurement of quantities" chapter (page 3) in "A treatise on electricity and magnetism" Maxwell uses, total length, $s=mt^{2}/{2r^{2}}$to show that $m=2sr^{2}/{t^2}$ is in units of Length cubed over Time squared. </p>
<p>This is the first I have encountered this way of look... | g18303 | [
0.05090613663196564,
0.030171353369951248,
-0.00613048393279314,
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0.04153348132967949,
-0.0071376021951437,
-0.013645218685269356,
0.05717623606324196,
0.0083... |
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