question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>This question may sound a bit dumb. Why can't we obtain the Hamiltonian of a system simply by finding $\dot{q}$ in terms of $p$ and then evaluating the Lagrangian with $\dot{q} = \dot{q}(p)$? Wouldn't we obtain then a Lagrangian expressed in terms of $t$, $q$ and $p$? </p>
<p>Why do we need to use </p>
<p>$$H(t, q... | g18581 | [
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<p>The Klein Gordon propagator is given (I use Peskin and Schroeder's conventions, if it matters...),</p>
<p>\begin{equation}
\frac{ i }{ p ^2 - m ^2 + i \epsilon }
\end{equation}
The photon propagator (using Feynman gauge) is
\begin{equation}
\frac{ - i \eta^{\mu\nu}}{ k ^2 + i \epsilon }
\end{equation}
The tim... | g18582 | [
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<p>Be $A_{ij}$ a symmetric matrix. Then I can easily write
$$
\int \exp\left(-\frac{1}{2}\sum_{i,j}x_i A_{ij} x_j+\sum_{i} B_i x_i\right)\; d^nx=
\sqrt{(2\pi)^n}\exp\left\{-\frac{1}{2}\mathrm{Tr}\log A\right\}\exp\left\{\frac{1}{2}\vec{B}^{T}A^{-1}\vec{B}\right\}.
$$
and this formula holds both in the finite and inif... | g18583 | [
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<p>Recently I read a paper using $$\pi_4(SU(2))=\mathbb{Z}_2.$$ Do you have any visualization or explanation of this result? </p>
<p>More generally, how do physicists understand or calculate high dimension homotopy group?</p> | g18584 | [
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<p>I'm trying to understand the problem of the "inverted" oscillator, which has the following Hamiltonian:
$$
\hat{H}=\frac{\hat{p}^{2}}{2m}-\frac{k\hat{x}^{2}}{2}
$$</p>
<p>Suppose that a particle at the initial time is in a state</p>
<p>$$
\Psi(x)=\frac{1}{(\pi b^{2})^{\frac{1}{4}}}\exp^{-x^{2}/2b^{2}}
$$</p>
<p>W... | g18585 | [
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<p>While directly diagonalize graphene's tight binding Hamiltonian, which is numerical. We have to use a finite-sized graphene.</p>
<p>So how to deal with boundary conditions? The usual solutions are zigzag or armchair condition, but to make our model more realistic to real infinite graphene plane, how about using per... | g18586 | [
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<p>I think I've got this figured out but wanted to make sure I'm doing this right. </p>
<p>Working with operators that satisfy bosonic commutation relations $[b,b^\dagger] = 1$, I define a very general unitary transformation on them:</p>
<p>$$\gamma^\dagger = ub^\dagger + vb + w$$
where the coefficients $u$, $v$, and... | g18587 | [
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<p>In the free electron approximation, a Bloch state $|k\rangle$ is the linear superposition of free plane wave states $\sum_G C_G(k) |k+G\rangle$, where $G$ are the conjugate lattice. Since the coefficients $C_G$ varies with $k$, under an electric field the electron will not merely shift inside the same band, but will... | g18588 | [
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<p>Some sources state that when the mass of a quark goes to zero, it allows for Spontaneous Breaking of Chiral Symmetry and gets a constituent mass of about $200\, \mathrm{MeV}$.</p>
<p>Other sources state that when the masses of the light quarks go to zero, so does the pion mass. In this case, the explicit breaking o... | g18589 | [
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<p>If Spaceship 1 is traveling at speed $.5c$ relative to Earth, and Spaceship 2 is traveling at speed $.3c$ relative to Earth in the same direction, what does Spaceship 2 see Spaceship 1's speed as?</p>
<p>I don't think it is quite as simple as $.5c-.3c = .2c$.</p> | g348 | [
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<p>Suppose we have a monochromatic light beam. We put an obstacle between source and observer and remove it repeatedly by certain frequency such that observer sees an oscillating intensity of light. Will the observer see different frequencies or only the original frequency? Does the energy-time uncertainty principle ap... | g575 | [
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<p>A two dimensional infinite "V"-shape wedge conductor is earthed, wherein $\beta$ is the intersection angle. We can solve Laplace equation so as to get the electric potential inside the "V" zone, as is shown in the figure. $a_n$'s are actually determined by some other boundary condition far away from origin and we ju... | g18590 | [
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<p>If space is warped by objects in space, and black holes are made of infinitely heavy objects, can space be torn by black holes?</p> | g18591 | [
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<p>Due to the mass-energy equivalence, both matter and EM radiation bend spacetime, and both are capable of forming singularities (black hole, white hole/kugelblitz). In light of this, why do photons traveling from the most distant reaches of the observable universe not lose energy due to the gravitational radiation th... | g18592 | [
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<p>When moving faster, we experience time dilation and space contraction. We often state that a photon does not travel through time, i.e. if it where capable of observation, it would not experience time.</p>
<p>But how about space? Would the photon experience a two-dimensional space? </p> | g361 | [
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<p>Is there a reason why a EM wave reflects invertedly when it meets a boundary point with a greater index of refraction. In the case of ropes, if remember correctly, the reason why it inverts is to agree with Newtons third law. But, its unclear why for EM waves, upon reflection, a phase change of $\pi$ occurs.</p> | g18593 | [
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<p>Can <a href="http://en.wikipedia.org/wiki/Spin_%28physics%29" rel="nofollow">spin</a> of a particle or a group of particles become infinity? Explain plz. Is there any representation for spins like dot(for $S=0$) and arrow (for $S=1$)? If so what for $S= \infty$?</p> | g18594 | [
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<p>Do magnets attract electrons? I don't think so, but maybe in certain cases they can be? I guess it would depend on what direction the velocity and magnetic field is in so that the force acting on the charge is towards one of the poles but I haven't tried that yet. </p>
<p>If an electron is shot between a north pole... | g18595 | [
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<p>When a charge is accelerated, it radiates and loses kinetic energy. This can be modeled by having another force act on the charge, which is proportional to the derivative of the acceleration. So if this force is included the equations of motion, we end up with 3rd order equations. Is this allowed? Why don't we need ... | g18596 | [
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<p>I get <a href="http://physics.stackexchange.com/questions/3489/what-is-quantum-entanglement">quantum entanglement</a> but I don't quite get how one would go about generating two complementary particles that are entangled (a photon and its entangled sibling, an electron and its entangled sibling, etc.).</p>
<p>Can a... | g18597 | [
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<p>Pictured below is a standalone steamer used for foaming and heating milk for milk-based coffee drinks like cappuccinos and lattes. They can be filled with water up to the pressure relief incorporated in the handle which is about 32 US fluid oz (1L) worth.</p>
<p>Once the pressure relief valve starts kicking in, a c... | g18598 | [
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<p>Here is a simple point about the standard interpretation of the Klein-Gordon equation that for the life of me I've never been able to understand:</p>
<blockquote>
<p>Why would the existence of true negative energy states result in an infinite downward cascade of photon emissions?</p>
</blockquote>
<p>I believe i... | g18599 | [
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<p>This is a follow-up question to <a href="http://physics.stackexchange.com/q/12353/293">D'Alembert's Principle and the term containing the reversed effective force</a>.</p>
<p>From the second term of Eq. (1.45)
$$\begin{align*}
\sum_i{\dot{\mathbf{p}}_i \cdot \delta{\mathbf{r}_i}} &=
\sum_i{m_i\ddot{\mathbf{r}... | g18600 | [
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<p>Such a device consists of a sensor pad that is placed under a mattress:</p>
<p><img src="http://i.stack.imgur.com/OMVui.gif" alt="sesnor pad placing under a mattress"></p>
<p>What's the principle it uses to detect breathing?</p> | g18601 | [
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<p>Can we make light slower by applying optical trapping (I mean applying laser beam to lower the speed of light)?</p> | g18602 | [
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<p>I want to know the major difference between <a href="http://en.wikipedia.org/wiki/Oscillon" rel="nofollow">oscillon</a> and <a href="http://en.wikipedia.org/wiki/Soliton" rel="nofollow">soliton</a> in terms of radiating energy with respect to time and position. And what about their localization?</p> | g18603 | [
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<p>Atomic bombs make mushroom. It's well-known and we've seen many images of atomic experiments around the world. My question is, why mushroom? Does TNT and Grenade also make mini-mushrooms? Is it a physical attribute of explosion, or something specific to atomic energy?</p>
<p>Thanks</p> | g18604 | [
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<p>$$\frac{d}{dt}\int \limits_{A} \mathbf B d \mathbf A = \int \limits_{A} \left( \frac{\partial \mathbf B}{\partial t} + \mathbf v (\nabla \cdot \mathbf B ) + [\nabla \times [\mathbf v \times \mathbf B ]\right)d\mathbf A, $$
where $d \mathbf A$ - infinitesimal element of the vector of a surface $\Sigma$ bounded by cir... | g18605 | [
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<p>Does the <a href="http://en.wikipedia.org/wiki/Oberth_effect" rel="nofollow">Oberth effect</a> only apply when in orbit of a planet or would a rocket generate more and more thrust (if kept on) even in deep space?
Wikipedia explains that the faster the rocket goes, the more useful energy it generates. But since speed... | g18606 | [
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<p>I'm working on a problem regarding two objects with the same kinetic energy.</p>
<blockquote>
<p>Two objects with masses of $m_1$ and $m_2$ have the same kinetic energy are both moving to the right. The same constant force F is applied to the left to both masses. If $m_1=4m_2$, the ratio of the stopping distance ... | g18607 | [
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<p>I'm looking for any resources that would help me model the behavior of objects moving in fluid. My end goal is to be able to describe the motion of irregularly shaped objects in a river environment. Any recommendations are welcome. </p> | g18608 | [
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-0.009356810711324215,
0.023467404767870903,
-0.039455171674489975,
0.05179261788725853,... |
<p><img src="http://i.stack.imgur.com/OZeCm.jpg" alt="enter image description here"></p>
<p>As seen above, I don't follow how you figure out those planes. It seems they're not using the origin labeled. I'm not really sure I understand spatially what's going on in the left figure so let's focus only on the right figure... | g18609 | [
0.006252834107726812,
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0.006... |
<p>I'm looking to get some data on the <a href="http://en.wikipedia.org/wiki/Lambda_transition" rel="nofollow">lambda transition</a> of $\require{mhchem}\ce{^4He}$. I need the data points of the specific heat vs. temperature graph, if that makes sense.</p> | g18610 | [
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<p>EM radiation seems to come from two different sources:</p>
<ol>
<li>According to the Maxwell, by acceleration of electrons</li>
<li>According to the Bohr, by jumping of electrons between energy levels?</li>
</ol>
<p>Are these two, completely different things?</p> | g18611 | [
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-0.... |
<p>Why is $I_0\sin\left(\dfrac{\pi}{2}-\omega t\right)$ not used as a solution of the purely capacitive circuit? </p>
<p><strong>My Research</strong> </p>
<p>I've searched on Google, Phys.SE and many school level Physics books but couldn't find the answer. If the reader thinks that the question can be answered wi... | g18612 | [
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<p>A question I have found that, <a href="http://www.physics4all.com/questions/after-a-black-hole-evaporates-what-happens-to-the-singularity/" rel="nofollow">in a question</a> it is saying that what will be happened to the singularity when black hole evaporates. But recently I have checked that Stephen Hawking denied t... | g18613 | [
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<p>Can someone give me a reference which gives the <a href="http://www.google.com/search?as_q=%22rotation+curve%22&as_epq=andromeda+galaxy" rel="nofollow">rotation curve of the Andromeda galaxy</a> (as well as its radius)?</p> | g18614 | [
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<p>The wave function determines the probabilities of an outcome, and the wave function is parameterized "shaped" depending on the measurement set.</p>
<p>i.e. two different measurement set would lead to two different wave function description.
Let's call A measurement set=>wave function A, and B measurement set=>wave ... | g18615 | [
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<p>This is for a homework assignment so it would be great to have a hint or an explanation of how to do it. In a certain region in space, the electric field is given by </p>
<p>$\vec{E}(r) = \dfrac{A}{r} \hat{r} + \dfrac{B \sin{\theta} \cos{\phi}}{r} \hat{\phi}$</p>
<p>What is the charge density in the region?</p> | g18616 | [
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<p>Kind of an extension to <a href="http://physics.stackexchange.com/questions/425/whats-the-relationship-between-thermal-radiation-and-johnson-thermal-noise">this question</a>:</p>
<p>If you heat up an object, and put it in contact with a colder object, in an ideal insulated box, the heat from one will transfer to th... | g18617 | [
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<p><a href="http://en.wikipedia.org/wiki/Supersymmetry" rel="nofollow">Wikipedia</a> says:</p>
<blockquote>
<p><em>In particle physics, supersymmetry (often abbreviated SUSY) is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartne... | g18618 | [
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0.0222... |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/3327/optical-explanation-of-images-of-stars">Optical explanation of images of stars?</a><br>
<a href="http://physics.stackexchange.com/questions/35935/what-are-these-rays-that-appear-in-photograph-of-sun">... | g576 | [
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0.03974661976099014,
0.04852701723575592,
0.08131028711795807,
0.029098... |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/34113/could-the-heisenberg-uncertainty-principle-turn-out-to-be-false">Could the Heisenberg Uncertainty Principle turn out to be false?</a> </p>
</blockquote>
<h1>Thought Experiment</h1>
<p>Ponder, for ... | g577 | [
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<p>Most of the time the normal force doesn't do any work because it's perpendicular to the direction of motion but if it does do work, would it be conservative or non-conservative?</p>
<p>For example, consider the following block-incline system where the incline is on a friction-less surface. Now as the block moves do... | g18619 | [
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<p>So I'm a game developer and I'm trying to understand some (extremely) basic facts of impact mechanics. I had read something entitled <em>Dynamics of Hand-Held Impact Weapons,</em> but it was a bit too complicated for my feeble mind, and really more than I needed to understand for a game.</p>
<p>I'm assuming as ave... | g18620 | [
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<p>If I'm not wrong, a bra, $ \langle \phi_n | $, can be thought as a linear functional that when applied to a ket vector, $| \phi_m \rangle$, returns a complex number; that is, the inner product it's a linear mapping that $\xi \rightarrow \mathbb{C} $; yet, exist a bra to each ket, and in discrete basis, the reverse ... | g18621 | [
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0.022663209587335587,
-... |
<p>Every reference I find says that they are <strong><em>"essentially"</em></strong> the same, which we all know really means that they are not the same, but different only by a some small amount that someone else other than me decides is negligible.</p>
<p>They also like to say that UT1 and UTC are the same, but if y... | g18622 | [
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<p>A large container of heated water is floating in space. How can I calculate leak rate if a small puncture is opened?</p>
<p>I'm assuming the water is kept at 298K and in the liquid state inside the chamber.</p>
<p>I guess that upon depressurization the water will immediately vaporize and then take some time to fre... | g18623 | [
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<p>From my book:</p>
<p>"A length of wire is cut into five equal pieces. The five pieces are then connected in parallel, with the resulting resistance being 2.00 Ω. What was the resistance of the original length of wire before it was cut up?"</p>
<p>I don't understand what exactly I'm looking for. What resister... | g18624 | [
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<p>I'm not sure which is the exact definition of a <a href="http://en.wikipedia.org/wiki/Casimir_invariant">Casimir operator</a>.</p>
<p>In some texts it is defined as the product of generators of the form:
$$X^2=\sum X_iX^i$$</p>
<p>But in other parts it is defined as an operator that conmutes with every generator o... | g18625 | [
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<p>A guy posted this problem on a forum:</p>
<blockquote>
<p><em>There is a bird sitting on a pole of height h. you throw a rock at it and the moment the rock leaves your hand the bird starts flying horizontally away from you at 10m/s, the rock passes the point where the bird was when you threw it and then reaches t... | g18626 | [
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<p><strong>My question :</strong> What does it mean when we arrive at <strong>negative</strong> values for <em>distance</em> variables like $\rho$ in cylindrical coordinates? (after some discussion here,I revised the question, at the end of the post)</p>
<p><strong>My specific example:</strong></p>
<p>For the motion ... | g18627 | [
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<p>Is it possible to answer my question definitely (assuming the monitor is perfect)? What would be the formula for calculating RGB values for a visible monochrome light with given wavelength?</p> | g18628 | [
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<p>In QFT , The action should be invariant under poincare symmetry $g_{\mu\rho}(x)=g_{\mu\rho}^{\prime}(x^\prime)$ . We can generalize this by considering theories invariant under conformal symmetry $g_{\mu\rho}^{\prime}(x^\prime)=A(x)g_{\mu\rho}(x)$ . If we consider more general variations ( Diffeomorphisms on spaceti... | g18629 | [
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<p>A spacecraft flies away from earth with a speed of 4.8 million meter per second relative to the earth, and then returns at the same speed. The spacecraft carries a clock that has been synchronised with a clock on earth, When the spacecraft returns to its starting point 1 year later as measured by the clock on earth,... | g18630 | [
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<p>According to wikipedia, commercially available super-capacitors in 2010 can store 30Wh/kg, with 85Wh/kg achieved in a lab in 2011. </p>
<p>Have any theoretical calculations been done to show what can be achieved as an upper limit to super-capacitors? </p> | g18631 | [
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<p>So I was taught that:</p>
<p>Kinetic Energy (of electron) = Energy (of photon) - Ionization Energy</p>
<p>If E(photon) = IE, then KE=0 of the electron. </p>
<p>What does this physically/theoretically mean? </p>
<p>My current thoughts/interpretation is that enough energy/force is applied to ionize the electron so... | g18632 | [
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<p>Extremal black holes are at zero temperature, hence they do not radiate.</p>
<p>my question is twofold:</p>
<p>1) <strong>is extremality of micro black holes a stable property?</strong> electric charge is quickly emitted from sub-atomic black holes due to Schwinger pair production, what about angular momentum? doe... | g18633 | [
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<blockquote>
<p><em>The problem is, "A metal can containing condensed mushroom soup has mass 220 g, height 11.0 cm and diameter 6.38 cm. It is placed at rest on its side at the top of a 3.00-m-long incline that is at 30.0° to the horizontal and is then released to roll straight down. It reaches the bottom of the incl... | g18634 | [
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<p>This is problem 3.18, from the book Statistical Mechanics by Pathria. Show that for a system in the canonical ensemble
$$\left\langle \left(\Delta E\right)^{3}\right\rangle =k^{2}\left\{ T^{4}\left(\frac{\partial C_{V}}{\partial T}\right)_{V}+2T^{3}C_{V}\right\} $$
Verify that for an ideal gas
$$\left\langle \left(\... | g18635 | [
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0.0230... |
<p>If the expansion of the universe is accelerating, doesn't that mean that the entire universe is a non-inertial frame of reference? And if so, what implications does this have (if any)?</p> | g18636 | [
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0.03... |
<p>I was told that the sum of linear and angular momentum is conserved. </p>
<p>Given that angular momentum's direction as a vector is completely arbitrary (I believe there is no physical reason for choosing the cross product going the way it does (that is, perpendicular <em>up</em> or <em>down</em>, it just matters t... | g18637 | [
0.025967907160520554,
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0.009469504468142986,
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<p>I'm confused by the many <a href="http://www.timeanddate.com/time/leapseconds.html" rel="nofollow">contradictory descriptions</a> I see about how UTC leap seconds are accounted for. I understand that there are <a href="http://www.usno.navy.mil/USNO/time/master-clock/leap-seconds" rel="nofollow">various ways</a> to h... | g18638 | [
0.04691804200410843,
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0.05850888043642044,
0.02629857... |
<p>To learn more about oscillatory motion which I am learning about in my high school physics class, I have created a computer model of a damped spring where the damping force is proportional to velocity. The graph of position vs time appears as expected, a sine wave with amplitude decaying over the course of time.</p>... | g18639 | [
0.1351851373910904,
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0.0621037594974041,
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-0.03101605735719204,
0.03914404660463333,
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0.03264787793159485,
-0.025... |
<p>You are 10 miles upstream from a dam. A 50 sq ft gate 30 feet above the lower water level is opened and water starts to flow from the dam. How long is it until your boat starts moving towards the dam? The lake is 300 feet across, 30 feet deep. You will be pulled towards the dam. Thinking I will need the volume ... | g18640 | [
0.05377267301082611,
0.042890243232250214,
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0.06284760683774948,
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0.025885403156280518,
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0.06838258355855942,
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<p>I am working on a complex configuration of magnets and every time I make an experiment something unforseen happens. Now I believe I could speed up the development by sitting down and calculating the configuration in all scenarios. My question is however: Is there any software that can help me to calculate the forces... | g18641 | [
0.04110380634665489,
0.013546345755457878,
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0.005191820207983255,
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0.007415447384119034,
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-0.016847733408212662,
0.00034610569127835333,
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0.023419072851538658,
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<p>Fluids (including natural gasoline/petroleum) have variable volume based on the ambient temperature for the same mass of fluid. So, really, the amount of gas that you're filling your car with depends on the temperature because it's not the <em>volume</em> of fuel that makes your car run, but the <em>mass</em> which ... | g18642 | [
0.044082894921302795,
0.004551500082015991,
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0.04523806646466255,
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<ol>
<li><p>What is the exact shape of the universe? I know of the balloon analogy, and the bread with raisins in it. These clarify some points, like how the universe can have no centre, and how it can expand equally everywhere in all directions.</p></li>
<li><p>But they also raise some questions, like if you are on th... | g653 | [
0.016476232558488846,
0.0016606596764177084,
0.0008793278248049319,
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0.009025094099342823,
0.010271169245243073,
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-0.06284714490175247,
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0.04528351500630379,
... |
<p>If $S=(v_{1},v_{2}......v_{n})$ is a basis for vector Space V, then every vector v in V can be expressed in the form of $v=c_{1}v_{1}+.......c_{n}v_{n}$ in an unique way.
Explain the significance of this result in Quantum Mechanics.
The answer to the first one is easy as its proof is based on Linear algebra and I ha... | g18643 | [
0.04045926406979561,
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0.028636785224080086,
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0.04707916080951691,
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0.01583302766084671,
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0... |
<p>I'm a graduate student in mathematics doing a bit of research in signal processing and Fourier analysis and I've come across a question that could probably be better answered by a physicist:</p>
<p>Is the phenomenon of octave equivalence (the psycho-acoustical sensation that pitch is subject to an equivalence relat... | g18644 | [
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0.008588900789618492,
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0.046617962419986725,
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0.020100481808185577,
0.018173139542341232,
0.0... |
<p>Since the movement of the air is induced by buoyancy, i. e. there's a constant force acting on the air, so I would expect the velocity to increase during ascent, much like an object falling down due to gravity accelerates.</p>
<p>But wouldn't a velocity gradient tear the column apart along the axis? Or would it jus... | g18645 | [
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0.01782839... |
<p>My Physics teacher stated that Pluto has a gravitational pull on objects on Earth, namely humans. Is this true? What is the free-fall acceleration of Pluto with respect to being on the Earth's surface? (i.e. the Earth's free-fall acceleration is 9.8 m/s*s).</p> | g18646 | [
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<p>Presumably there is a field theory of electromagnetism that classically gives trivial equations of motion, but when quantized shows interesting topological phenomena. I am talking about the Lagrangian density
$$\mathcal{L}=\epsilon^{\mu\nu\lambda\sigma} F_{\mu\nu}F_{\lambda\sigma}\propto \vec{E}\cdot\vec{B}$$
where ... | g18647 | [
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<p>In one book, I have got the following lines which I found myself unable to understand what is effective operator? The paragraph is given below:</p>
<blockquote>
<p><em>The weak interaction describes nuclear beta decay, and at low energy it is given
by an effective four fermion interaction. Since the effective o... | g18648 | [
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0.04531458020210266,
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<p>If I take a bottle of carbonated water and open the cap slightly to allow the gas to escape, there will be a sudden rush of bubbles foaming to the top of the liquid for a few seconds, then a partial lull for a few seconds, then another smaller rush of bubbles, then a smaller lull, etc, like a damped oscillation. Wh... | g18649 | [
0.04065559059381485,
0.027604632079601288,
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0.033174168318510056,
0.07000739127397537,
0.015... |
<p>The particular Something I had in mind here would be air bubbles that are pulled downwards against their buoyancy by a stream of water falling down a shaft. The work required to push those bubbles down must decrease energy somewhere else, so where does it come from? Is the stream's velocity reduced?</p> | g18650 | [
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-0.06218120828270912,
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0.01431992556899786,
0.016383541747927666,
0.0141259012... |
<p>In the loop-the-loop ride a car goes around a vertical, circular loop at a constant speed. The car has the mass of 230 kg and moves with the speed of 300 m/s. The loop-the-loop has a radius R=20 m. What would then be the magnitude of the normal force on the car when it is at the side of the circle moving upward? </p... | g18651 | [
0.021291611716151237,
0.020323019474744797,
0.00023871638404671103,
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0.033689018338918686,
0.026002557948231697,
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... |
<p>I am hoping someone can please give me an answer. I was at a dog park with my dog and son (hes 12), which is directly under high/loud voltage power lines. I went to brush some dirt of my sons neck when I felt his neck "buzzing". He didnt feel it. I felt all over him, and he was buzzing/vibrating all over his enti... | g578 | [
0.10113628953695297,
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... |
<p><img src="http://i.stack.imgur.com/8QAKW.jpg" alt="enter image description here"></p>
<p>I have spotted this on the rooftops of industries, workshops, etc.
I guess this is used for providing ventilation. This is not electrically powered. But then how does this work??</p> | g18652 | [
0.03353436663746834,
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0... |
<p>I've read that gravitational lensing bends the light; I am a little confused about black holes and why lensing produces different results. Black holes absorb light whereas lensing should have also done the same but why does gravitational force help bend the light?</p> | g18653 | [
0.03477002680301666,
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0.0694437175989151,
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0.0246... |
<p>I want to compute the components of the Riemann curvature tensor (for a case similar to the Schwarzschild solution) in 4 + 1 dimensions, but I want to use a higher-dimensional analogue of spherical coordinates. I first want to investigate a metric for the Euclidean case, i.e. "flat" space-time with no matter or ener... | g18654 | [
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0.08187346905469894,
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<p>In parametric down-conversion, it is said that a driving photon is converted into two entangled photons whose frequencies add up to the driving frequency. Yet in discussions about entanglement experiments, I have not seen anything about the frequency at the point of detection. What is the story here? Do you have pai... | g18655 | [
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0.019975576549768448,
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0.013971486128866673,
-0.04... |
<p>I'd like to know the exact context in which Paul Dirac introduced the Dirac delta function. What was the physical significance of the Dirac delta function when he first used it in Physics ?</p> | g18656 | [
0.04352541267871857,
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0.05921462923288345,
0.02403993... |
<p>If one defines force as the time derivative of momentum, i.e. by</p>
<p>$$
\vec{F}= \frac{d}{dt} \vec{p}
$$</p>
<p>how can this include static forces? Is there a generally accepted way to argue in detail how to get from this to the static case? If not, what different solutions are discussed?</p>
<p>Edit:
I sh... | g18657 | [
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0.011450691148638725,
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0.01319185458123684,
-0.0... |
<p>Stephen Hawking says in his latest book <a href="http://en.wikipedia.org/wiki/The_Grand_Design_%28book%29"><em>The Grand Design</em></a> that,</p>
<blockquote>
<p>Because there is a law such as gravity, the universe can and will create itself from nothing.</p>
</blockquote>
<p>Is it not circular logic? I mean, h... | g18658 | [
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-0.02207152359187603,
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0.02530987374484539,
0.031... |
<p>My question about the sum of the electrons' magnetic moments in a wire(<a href="http://physics.stackexchange.com/questions/134012/what-is-the-sum-of-the-electrons-magnetic-moments-in-a-wire?noredirect=1#comment274281_134012">What is the sum of the electrons' magnetic moments in a wire?</a>) had an answer which d... | g18659 | [
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0.03760622441768646,
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0.013786924071609974,
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0.02762744575738907,
-0.02889399789273739,
0.003... |
<p><a href="http://en.wikipedia.org/wiki/Antineutron" rel="nofollow">Wikipedia</a> explains: </p>
<blockquote>
<p><em>The antineutron was discovered in proton–proton collisions at the Bevatron (Lawrence Berkeley National Laboratory) by Bruce Cork in 1956, one year after the antiproton was discovered.</em></p>
</bloc... | g18660 | [
0.08335509151220322,
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0.05045267567038536,
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0.0084084747... |
<p>Right now, I'm looking at two straightforward derivations of the dispersion relations of electromagnetic waves in conductors, and they seem to disagree on the low frequency attenuation. Could someone supply some insight into the regimes of validity of these very basic methods?</p>
<h2><strong>In my electromagnetis... | g18661 | [
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-0.002898111240938306,
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0.03481458127498627,
0.028956830501556396,
0... |
<p>I'm trying to do an exercise in which you solve the Schrödinger equation for the hydrogen atom. Through the exercise, I've already shown that the wavefunction is:</p>
<p>$$ \psi_{n\ell m}(r,\theta,\varphi) = R_{n\ell}(r)Y^m_\ell (\theta,\varphi)$$</p>
<p>and that $Y^m_\ell (\theta,\varphi)$ are the spherical harmo... | g18662 | [
-0.0015367827145382762,
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0.057692766189575195,
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-0.010232296772301197,
-0.012947484850883484,
0.06872733682394028,
-0.0135106723755598... |
<p>To get an expansion of Helmholtz energy of </p>
<p>a) an ideal gas
b) a Van der waals gas </p>
<p>we must integrate </p>
<p>$\left ( \frac{\delta A }{\delta V} \right )_{T}=-P$</p>
<p>I saw the solution is :</p>
<p><img src="http://i.stack.imgur.com/Njbor.png" alt="enter image description here"></p>
<p>Can yo... | g18663 | [
0.03857125714421272,
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0.014871247112751007,
0.0036093173548579216,
0.... |
<p>I have 2 rigid bodies (from different materials) in a collision.</p>
<p>As you know I should have the <a href="http://en.wikipedia.org/wiki/Coefficient_of_restitution" rel="nofollow">coefficient of restitution</a> value to get the velocities after collision.</p>
<p>What is the information/values should I know abou... | g18664 | [
0.07533223181962967,
0.008976665325462818,
0.00016950219287537038,
0.012412524782121181,
0.09959274530410767,
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0.0474972240626812,
0.03478777036070824,
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0.03006882406771183,
-0.03... |
<p>While studying about Fresnel and Fraunhofer diffraction, I came across a statement which says that the fringes disappears and the image would take on the limiting shape of the aperture when wavelength goes to zero, which is the same as predicted by geometrical optics.
I don't understand the meaning of this, espe... | g18665 | [
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<p>I'm reading my textbook and it says the distance between two successive nodes is equal to $\frac{1}{2} \lambda$ in standing wave.</p>
<p><img src="http://i.stack.imgur.com/86Y0R.jpg" alt="enter image description here"></p>
<p>If $\lambda$ here means the wavelength of the standing wave shouldn't it be:</p>
<p>when... | g18666 | [
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-... |
<p>As we know magnetic phenomenon is a mere relativistic effect.My question is how to explain the magnetic induction in a relativistic manner?</p> | g18667 | [
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0.0261434... |
<p>When two vectors are sketched from a single point, the angle between them is θ. Show that the size of their vector summation is given in the expression: $ \sqrt{A^2 + B^2 +2ABcosθ} $. </p>
<p>Any suggestions on how one might tackle this one? </p> | g18668 | [
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<p>According to our physics textbook, the source of sound is vibration. According to our chemistry textbook, substances in solid state have molecules vibrating in a fixed position. So why can't I hear my desk or something?</p> | g854 | [
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... |
<p>Is any quantum state $|\psi\rangle$ possible such that the standard deviation $\sqrt{\langle\psi|(\Delta\hat{x})^2|\psi\rangle}$ of the position operator $\hat{x}$ is zero? If not, why? </p> | g18669 | [
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<p>We performed a laboratory, performing six releases of a sphere with angles $15^\circ,30^\circ,45^\circ,60^\circ,75^\circ,40^\circ$ a parabolic movement, took five distances for each angle, the initial velocity was calculated $3.025~\text{m/s}$.
Then doing 5 tosses of the sphere with an angle of $90^\circ$ times wer... | g18670 | [
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<p>I have the following problem: </p>
<blockquote>
<p>In nuclei, nucleons exists in nuclear energy levels and in atoms, electrons exist in atomic energy levels. The order of magnitude of nuclear energy is 1MeV whereas the energy of atomic energy levels is of the order 1eV. Use this info and the particle in the box m... | g18671 | [
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<p>The temperature or relic radiation is 2.7 K. Does it mean that any flying body sufficiently far from any stars will reach this temperature?</p>
<p>As I understand it, you can warm up water with sufficient amount or radiation with 2.7K spectrum and you can freeze helium even in presence of a source of photons with 1... | g18672 | [
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