question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
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<p>Recently I was taking a look at a <a href="http://www.youtube.com/watch?v=eGguwYPC32I" rel="nofollow">video</a> explaining the existence of fourth spatial dimension and thereupon that infinitely many spatial dimensions are possible. Also it showed that what Einstein told about time being a dimension itself was also... | g6657 | [
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<p>I am just wondering say if there is an expedition where some astronauts are sent to the moon, how do they choose the trajectory for the spaceshuttle (or whatnot)? I mean there are many possible trajectories depending on the initial velocity at which the vessel is launched. There must be some sort of optimal trajecto... | g6658 | [
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<p>I'm having a little trouble understanding Shor's algorithm - namely, why do we throw away the result f(x) that we get after applying the F gate? Isn't that the answer we need?</p>
<p>My notation:
$\newcommand{\ket}[1]{\left|#1\right>}$</p>
<p>$F(\ket x \otimes \ket0) = \ket x \otimes \ket{f(x)}$ , </p>
<p>$f(x... | g6659 | [
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<p>Let $| 0 \rangle$ and $| 1 \rangle $ be the states of qubit. Let $\hat{\sigma_x}$, $\hat{\sigma_y}$, $\hat{\sigma_z}$ be Pauli matrices:
$$
\hat{\sigma}_{x} = \left( \begin{array}{cc} 0 & 1 \\ 1 & 0 \end{array} \right), \;\;\;
\hat{\sigma}_{y} = \left( \begin{array}{cc} 0 & -i \\ i & 0 \end{arr... | g6660 | [
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<p>I was wondering if I can say to a layman that <strong>"upon throwing the ball on a wall an enormously large number of times, there is a small probability that the ball will go through the wall"</strong>, while explaining quantum tunneling (alpha decay example is abstract and artificial for a layman).</p>
<p>My doub... | g6661 | [
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<p>I am calculating the propagator of the free particle on a sphere : $K(\theta_f \phi_f t_f; \theta_i \phi_i t_i)$. The wavefunctions in this case are the spherical harmonics $Y_{lm}(\theta, \phi)=\frac{e^{im \phi}}{\sqrt{2 \pi}}{\Omega(\theta)}$. So the kernel is
$$K=\sum e^{\frac{-i E_n (t_f-t_i)}{\hbar}} \frac{e^{... | g6662 | [
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<p>I understand that gravity is viewed as flowing as a river pushing objects down on the body of a planet. If that is the case and earth is a sphere, where does the gravity go when it hits the center of the planet? Does it get compressed? Is it destroyed? </p> | g6663 | [
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<p>I am trying to calculate the heat loss of an aquifer. An aquifer is basically a hot-cold storage $80\: \mathrm{meters}$ under the ground. It is an open system and utilizes groundwater. By pumping for example warm water ($70\: \mathrm{C}$) under the ground this can be stored for half a year and be used in the winter ... | g6664 | [
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<ol>
<li><p>In a previous question, I was given an answer:
"A quick Google suggests that the triple point of Hydrogen is 13.8K and the triple point of Neon is 24.6K, so neither can exist as liquids at temperatures low enough to form BECs." CVB Note. It was inferred later in the answer, that a gas can never exist as a ... | g6665 | [
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<p>$erg$ is an Energy unit, which means that it is equal to $\frac{mass{\cdot}distance^2}{time^2}$. If I want to calculate the kinetic energy of a body in erg units, in what units should the distance and mass be?</p>
<p>By "what units" I mean $kg$ or $gram$ and $cm$, $m$ or $km$ not $inch$ vs. $m$.</p> | g6666 | [
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<p>How to show that $\hat {S}$-operator must be lorentz-invariant operator?</p>
<p>$$
|\Psi (t)\rangle = \hat {S} | \Psi (0) \rangle , \quad \hat {S} = \hat {T}e^{-i\int \hat {H}_{I}d^{4}x}.
$$
I have read that this result follows from the unitarity of the Poincare group operator $U_{0}(\Lambda , a)$ and the covarianc... | g6667 | [
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<p>Why light travels in straight paths here on earth? What's the real cause that makes light photons to go in a straight line? and what are the factors that could change the path of light externally? (I'm excluding reflection, refraction and deviation here)</p> | g6668 | [
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<p>In school-level tasks, when (almost) all substances are linear, homogeneous and isotropic, we have $D=\epsilon E$, $H=B/\mu$ and thus Maxwell "in material" equations (1) say how $E$ and $B$ depend on time given known dependence of $\rho$ (free charge density) and $j$ (free current density). Here they are in CGS unit... | g6669 | [
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<p>I have a question about cosmology. At popular level people explain the time of decoupling of matter and radiation as the moment when temperature falls enough for nuclei and electrons to recombine into atoms. People say that the Universe became “transparent”. The photon cross section by an electro-neutral... | g6670 | [
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<p><a href="http://en.wikipedia.org/wiki/Potts_model">Wikipedia</a> writes:</p>
<blockquote>
<p>In statistical mechanics, the Potts
model, a generalization of the Ising
model, is a model of interacting spins
on a crystalline lattice.</p>
</blockquote>
<p>From combinatorics conferences and seminars, I know tha... | g6671 | [
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<p>Two bodies with similar/different mass orbiting around a common barycenter.</p>
<ul>
<li>What is force between them, where $F_{12}$ is the force on mass 1 due to its interactions with mass 2 and $F_{21}$ is the force on mass 2 due to its interactions with mass 1?</li>
<li>What is relation between $F_{12}$ and $F_{2... | g6672 | [
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<p>I was just wondering what happens in a circuit in terms of different types of energy transformations. If you apply a voltage to a circuit then electrons start moving (very slowly). Since the electrons want to flow to the positive terminal they will have electrical potential. But the electrons are also now moving onc... | g6673 | [
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<p>Under <a href="http://en.wikipedia.org/wiki/Active_and_passive_transformation" rel="nofollow">active transformation</a>, the particle moves. On the other hand, for a passive one, the coordinate is just relabel. </p>
<p>I've read that the passive one will not affect the potential energy and the active one will chang... | g6674 | [
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<p>I want to make a home experiment where I roughly explain the phenomenon of water remaining in a straw if you close one end of the straw.</p>
<p>So I'm thinking that the weight of the water is pulling it down, but the pressure underneath the straw is keeping it up. If there was no finger on the top, the pressure wou... | g6675 | [
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<p>I've just read <a href="http://image.gsfc.nasa.gov/poetry/ask/a11024.html">here</a> that:</p>
<blockquote>
<p>Equatorial radius = 6378.16 kilometers. Polar radius = 6356.78
kilometers, so the difference in circumference is 71.1 kilometers. It
is not a perfect sphere, but kind of pear-shaped.</p>
</blockquote>... | g6676 | [
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<p>I'm a bit confused about the difference between these two concepts. According to Wikipedia the <a href="http://en.wikipedia.org/wiki/Fermi_energy">Fermi energy</a> and <a href="http://en.wikipedia.org/wiki/Fermi_level">Fermi level</a> are closely related concepts. From my understanding, the Fermi energy is the highe... | g916 | [
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<p>I'm a bit confused about how solar cells work. </p>
<p>My understanding is that there is a p-n junction. A photon is absorbed which creates an electron-hole pair, and the idea is to separate the electron and hole before they recombine in order to produce current.</p>
<p>n-type materials are doped with extra electr... | g6677 | [
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<p>In <em>Gravitation and Cosmology</em>, S.Weinberg states the following:</p>
<p>$$\Lambda_{\epsilon}^{\alpha}\Lambda_{\zeta}^{\beta}\Lambda_{\kappa}^{\gamma}\Lambda_{\lambda}^{\delta}\epsilon^{\epsilon \zeta \kappa \lambda} \propto \epsilon^{\alpha \beta \gamma \delta} $$</p>
<p>and the argument is that the left-ha... | g6678 | [
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<p>When 'constructing' the usual de Sitter space in $\mathcal{M^5}$ by invoking the contraint $-X^{2}_{0} +X^{2}_{1} +X^{2}_{2} +X^{2}_{3} + X^{2}_{4} = \alpha^2$ we quickly see that we end up with a hyperboloid suspended in Minkowski space.</p>
<p>Most literature continues by invoking different coordinates on this em... | g6679 | [
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<p>Consider a nanoscopic wire (with radius $R$) of superconducting material. The wire lies along the $z$-axis and a magnetic field $\mathbf{H}_a = H\mathbf{e}_z$ is applied. The magnetic field is too weak to destroy superconductivity or to induce vortices and we assume that the superconducting parameter $\psi$ can be p... | g6680 | [
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<p>polarization could be easily imagined in classical model: direction of E vector. is there any simple image for polarization of single photon?</p> | g6681 | [
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<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/33916/what-is-the-escape-velocity-of-a-black-hole">What is the escape velocity of a Black Hole?</a> </p>
</blockquote>
<p>This is gravitational speed for earth: $$v=\sqrt {g_{e}r}.$$
What is gravitation... | g281 | [
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<p>I have read in a couple of places that $\psi(p)$ and $\psi(q)$ are Fourier transforms of one another (e.g. Penrose). But isn't a Fourier transform simply a decomposition of a function into a sum or integral of other functions? Whereas the position and momentum wavefunctions are essentially different but related. ... | g6682 | [
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<p>An object of a given mass falls from an unknown height. If the force exerted by the object on contact with the ground is known, how would you ascertain the height from which the object fell?</p> | g678 | [
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<p>Previous posts such as <a href="http://physics.stackexchange.com/questions/907/type-of-stationary-point-in-hamiltons-principle">this</a> ask about types of stationary point in Hamilton's Principle. There is, however, another aspect to discuss: the question as to whether the extremal path is unique.</p>
<p>One geome... | g6683 | [
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<p>I've read material claiming it comes from the Higgs boson fails while others claim it is from the tensions of quarks in the gluon field... I am only a 15 year old kid in high school so please "dumb it down" </p> | g229 | [
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0.07312067598104477,
0.030241504311561584,
0.0014990016352385283,
0.001817945740185678,
-0.06095393747091293,
-0.04469557851552963,
-0.010041572153568268,
-0.011938072741031647,
0.00... |
<p>After watching the first episode of wonders of the solar system, one question came up which is not explained.</p>
<p>Bryan Cox says that ultimately the universe will be devoid of matter, so not even a single particle will actually exist. This however, seems to be a state where entropy is zero. There is no entropy s... | g6684 | [
0.010690604336559772,
0.01702021434903145,
-0.0016177799552679062,
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0.024592433124780655,
0.040515750646591187,
-0.052518509328365326,
0.04906249791383743,
0.03126829117536545,
-0.08117521554231644,
0.009911193512380123,
-0.012152863666415215,
0.038692351430654526,
0.... |
<p>what is physics behind Water drops during falling from a tap.
<a href="http://upload.wikimedia.org/wikipedia/commons/b/bd/Water_drop_animation_enhanced_small.gif" rel="nofollow">water drop animation</a></p>
<p>A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces.</... | g6685 | [
0.045585863292217255,
0.04974149167537689,
-0.027187153697013855,
0.0062028891406953335,
0.02807764895260334,
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0.0029752119444310665,
0.004549921955913305,
-0.04613543301820755,
-0.01585892215371132,
-0.011262479238212109,
0.07622738182544708,
0.060... |
<p>In our de Sitter phase, the cosmological constant is tiny. $10^{-123}M_P^4$. Suppose there is another phase with a lower vacuum energy. Is de Sitter phase still stable? The tunneling bubble radius has to exceed the de Sitter radius. Suppose a metastable decay to such a bubble happened. Take that final state, and evo... | g6686 | [
-0.002514463849365711,
0.053894806653261185,
0.01987043023109436,
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0.008575476706027985,
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0.005713081918656826,
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-0.012754766270518303,
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0.0020955728832632303,
0.014613623730838299,
0.... |
<p>What is the difference between <a href="http://en.wikipedia.org/wiki/Parallel_universe" rel="nofollow">parallel universe</a> and <a href="http://en.wikipedia.org/wiki/Multiverse" rel="nofollow">multiverse</a>? Is it parallel universe or universes?</p> | g6687 | [
0.003683242481201887,
0.051624786108732224,
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0.02839077264070511,
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0.0075902328826487064,
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0.0... |
<p>I'm beginning learn quantum mechanics. As I understand, <em>state</em> is a map $\phi$ from $L^2(\mathbb R)$ such that $|\phi|^2$ describes probability density of a particle's position. By integrating $|\phi|^2$ we can get the probability of a particle's position in a subset of $\mathbb R$.</p>
<p><strong>My questi... | g6688 | [
-0.015267000533640385,
0.016387660056352615,
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-0.043581537902355194,
0.020980829373002052,
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-0.023688288405537605,
... |
<p>Looking at the classical Doppler effect there is one generalized equation, and they have a velocity of source, and observer.
In the relativistic version, there is only one velocity taken into account, and, in my book, there are two separate equations for receding and approaching sources.
These two formulas are le... | g6689 | [
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0.04996... |
<p>I'm reading <em>Music, Physics and Engineering</em> (H. Olson 1967), and contains models of musical instruments as electric circuits, i.e. uses the analogies between electrical circuits and mechanical (mass-spring-dashpot) systems. (The idea is that the reader will be more familiar with the electrical circuit repre... | g6690 | [
0.015793388709425926,
0.010645151138305664,
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0.04052389785647392,
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0.003727283561602235,
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<p>Being interested in the mathematical theory, I was wondering if there are up-to-date, nontrivial models/theories where delay differential equations play a role (PDE-s, or more general functional differential equations).</p>
<p>It is clear that</p>
<ul>
<li>in biological (population) models usually pregnancy introd... | g6691 | [
0.04166188836097717,
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0.07848865538835526,
0.00345... |
<p>It is clear that if you push on some object, there is reaction of the same force. But is it the same energy? Thanks a lot.</p> | g6692 | [
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<p>My friend puzzled me with this: Say I have a container that contains a vacuum and another container inside. And this inner container contains air. Also the outer container material is just barely strong enough to not implode due to the vacuum. If I remove the air from the inner container to outside of the outer c... | g6693 | [
0.016713734716176987,
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<p>In Higgs mechanism, we take the combination of LH $SU(2)$ doublet and RH singlet along with Higgs doublet so that the overall weak hypercharge and weak isospin is zero to be $SU(2) \times U(1)$ invariant. I am pretty confident about my understanding on weak hypercharge calculation. But it seems weak isospin calculat... | g6694 | [
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<p>I'm confused about the Second Law of Thermodynamics. The Second Law of Thermodynamics prohibits a decrease in the entropy of a closed system and states that the entropy is unchanged during a reversible process.</p>
<p>Then why do we say that $\Delta S = \int_a^b{\frac{dQ}{T}}$ for a reversible process? Doesn't the ... | g6695 | [
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<p>I have a mutipartite system; the subsysyems having interacted. Now, to find reduced density matrix for any single subsystem, is the order of partial traces(over remaining subsystems) important?</p> | g6696 | [
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<p>I'm getting some weird results from a calculation I'm doing and quite honestly, I'm pretty sure it's my fault. I do have an apparatus involved for the experimental process for my lab but I don't think it's that. I've come to the conclusion that my notes do not contain the right equations for these calculations. </p>... | g6697 | [
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<p>What is relation of fraction of binary stars with spectral class (mass)? For example, how many binary stars are among O,B,A,F,G,K,M stars separately?</p> | g6698 | [
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<p>Why do the electrons revolve around the nucleus in circular motion not like the earth which revolves around the sun in an eliptical motion?</p> | g6699 | [
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<p>I'm not expecting any rigor in the following and the answers...since we're dealing with Dirac deltas in the context of QFT. </p>
<p>Consider the integral </p>
<p>$$
\int d^4q\ \Theta(q_0)\Theta(p_{3,0}+q_0)\ \delta((p_3+q)^2)\delta((q^2))\frac{1}{(q+p_2)^2-m^2+i\epsilon}
$$</p>
<p>The $p$'s and $q$'s are all Mink... | g6700 | [
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<p>We consider a scalar theory in a $1+D$ dimensional flat Minkowski
space-time, with a general self-interaction
potential, whose action can be written as
\begin{equation}
A=\int dt\, d^D\! x \left[\frac12(\partial_t\phi)^2-\frac12(\partial_i\phi)^2
-U(\phi)\right] \,,
\end{equation}
where $\phi$ is a real scalar field... | g6701 | [
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0.004685163963586092,
0.019... |
<p>Suppose there is the following situation:</p>
<blockquote>
<p>Blocks $A$ and $B$, with masses $m_A$ and $m_B$, are connected by a
light spring on a horizontal, frictionless table. When block $A$ has
acceleration $a_A$, then block $B$ has, by Newton’s second and third
laws, acceleration $-a_A\frac{m_A}{m_B}$... | g6702 | [
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0.014134101569652557,
0.023... |
<p>What effects would occur if the earth's core goes cold? Would the planet stay liveable after this happens?</p> | g6703 | [
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<p>So I collected current and voltage data from a simple circuit with a power source and a resistor, using a multimeter. I created a graph for this data using excel and got the y-intercept (which is basically the value of the current when the voltage was zero) and found it to be a small negative number:</p>
<p><img sr... | g6704 | [
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0.024537499994039536,
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-0.... |
<p>I'm specifically looking for Schrödinger's Cat states involving superpositions of two, or if it's been done more, coherent states, i.e. monomodal states of the form $$|\psi\rangle=a|\alpha\rangle+b|\beta\rangle.$$
What states of this form have been produced in experiment? How even can the weights be? What regions of... | g6705 | [
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0.04332263022661209,
0.... |
<p>In an intro to GR book the Ricci tensor is given as:</p>
<p>$$R_{\mu\nu}=\partial_{\lambda}\Gamma_{\mu \nu}^{\lambda}-\Gamma_{\lambda \sigma}^{\lambda}\Gamma_{\mu \nu}^{\sigma}-[\partial_{\nu}\Gamma_{\mu \lambda}^{\lambda}+\Gamma_{\nu \sigma}^{\lambda}\Gamma_{\mu \lambda}^{\sigma}]$$</p>
<p>I have gotten to the po... | g6706 | [
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0.05115281790494919,
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0.007394233252853155,
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0.022210227325558662,
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... |
<p>As far as i know, microwave (used in microwave oven) and wifi all operate on the same frequency, but why microwave can heat the foods while wifi wave can't?</p> | g230 | [
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<p>I am trying to model a system in which cubes of about 2 cm in size are floating in a circular water thank of about 30 cm in diameter. The cubes move around under the influence of the fluid flow induced by four inlets that point toward the center of the tank, and are located at the positions $0$, $\pi/2$, $\pi$, and ... | g6707 | [
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0.003967116586863995,
0.03991098329424858,
0.0020114739891141653,
0.0... |
<p>What is similar from dynamic point of view between book lying stationary on horizontal table and rain drop falling down with constant speed?</p>
<hr>
<p>I can find two similarities</p>
<ol>
<li>Force acting on them is equal</li>
<li>Both have horizontal velocities zero</li>
</ol>
<p>Is there any other similariti... | g6708 | [
0.08566496521234512,
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0.03369573503732681,
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0.003107412252575159,
0.029... |
<p>The optical pumping experiment of Rubidium requires the presence of magnetic field, but I don't understand why. </p>
<p>The basic principle of pumping is that the selection rule forbids transition from $m_F=2$ of the ground state of ${}^{87} \mathrm{Rb}$ to excited states, but not the other way around ($\vec{F}$ is... | g6709 | [
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0.003274286864325404,
0.013052834197878838,
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-0.01768... |
<p>I'm reading John Taylor's Classical Mechanics book and I'm at the part where he's deriving the Euler-Lagrange equation. </p>
<p>Here is the part of the derivation that I didn't follow: </p>
<p><img src="http://i.stack.imgur.com/7B0wm.jpg" alt="enter image description here"></p>
<p>I don't get how he goes from 6.9... | g6710 | [
0.10287104547023773,
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-0.027077244594693184,
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0.026996321976184845,
0.05625439062714577,
... |
<p>Why do we get the same differential equations from both principles? Surely there is a fundamental connection between them? When written out, the two seem to have nothing in common. </p>
<p>$$\sum _i ( \mathbf F _i - \dot{\mathbf p}_i) \cdot \delta \mathbf r _i = 0$$</p>
<p>$$S[q(t)] = \int ^{t_2} _{t_1} \mathcal L... | g6711 | [
0.022190038114786148,
0.020322108641266823,
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0.07606631517410278,
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0.01071872003376484,
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0.02530806139111519,
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0.04693608358502388,
-0.03... |
<p>I would like to hear the best arguments for and against the <a href="http://en.wikipedia.org/wiki/Many-worlds_interpretation" rel="nofollow">Many Worlds interpretation</a> of QM.</p> | g6712 | [
0.006133685354143381,
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<p>Recently in class we went over Newton's Third Law. In the book they put an example of hitting a punching-bag with your fist and hitting a piece of paper, or an object with much less mass. It's clear that you cannot exert more force on the paper than what the paper can exert on you, otherwise it might stop your fist ... | g6713 | [
0.03631259873509407,
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0.00619892030954361,
0.04069028049707413,
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0.027882177382707596,
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-0.02703762799501419,
0.02797926962375641,
-0.04688466712832451,
-0.01061499398201704,
0.0030687... |
<p>An electrostatic field is characterized by the fact that it depends only by $r$, isn't it? If it is true, I don't understand why this expression, given in cylindrical coordinates,</p>
<p>$${\bf E(r)}=\frac{\alpha}{z^2}{\bf u_r}-2 \frac{\alpha r}{z^3}{\bf u_z} $$</p>
<p>rapresents an electrostatic field.</p> | g6714 | [
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... |
<p>According to <a href="http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html" rel="nofollow">NASA’s Saturn fact sheet</a>, Saturn has a density considerably less than water and the lowest density of the major planets. What compositional and/or structural mechanisms are theorised to be the cause of this low d... | g6715 | [
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0.0210... |
<p>The attenuation of a wave through a medium can be modeled by the <a href="http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law" rel="nofollow">Beer-Lambert Law</a> using an <a href="http://en.wikipedia.org/wiki/Attenuation_coefficient" rel="nofollow">attenuation coefficient</a>. If $I$ is the intensity, and $I_r$ ... | g6716 | [
0.019607963040471077,
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<p>How do you find the net <a href="http://en.wikipedia.org/wiki/Mechanical_advantage" rel="nofollow">Mechanical Advantage</a> (MA) of two joint machines. Do you add or multiply the individual MA?</p>
<p>Suppose I have two sets of wheel and axle connected by a fixed pulley. Each of the wheel has a radius of 100cm and ... | g6717 | [
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0.015184622257947922,
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<p>There's another question on physics.SE whose <a href="http://physics.stackexchange.com/a/2140/5971">answer</a>, if I have understood it correctly, explains that the farther the points are in space the faster they are moving away from each other.</p>
<blockquote>
<p>Actually, the apparent speed with which two of t... | g6718 | [
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<p>We have a basic equation $F=ma$. Now can we change this equation to another variable and start a new era in physics? If so, then how? If not, then why?</p> | g6719 | [
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<p>In terms of General relativity we have as a matter of principle that anything that has inertial mass contributes to gravity. All forms of potential energy have inertial mass, it follows that the potential gravitational energy of a star must contribute to the gravitational field of that star. Depending on the amount ... | g6720 | [
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<p>Since someone commented this on this question(<a href="http://physics.stackexchange.com/questions/93345/what-is-the-probability-of-ice-in-boiling-water?noredirect=1#comment190832_93345">What is the probability of ice in boiling water?</a>), I would like to ask what is the probability that all the air ends up in the ... | g6721 | [
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<h3>Situation</h3>
<p>Police shooting a <a href="http://en.wikipedia.org/wiki/LIDAR_speed_gun" rel="nofollow">laser device</a> into his rearview mirror, bouncing off the oncoming car license plate. I want to know if this second surface mirror, and the symmetric system--laser gun to mirror to license plate of moving ... | g6722 | [
0.02873593010008335,
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<p>I need to find the Lagrangian for charged particles in EM fields considering relativistic effects. Is action integral Lorentz invariant.
$$A = \int_{t_1}^{t_2} L (q_i, \dot q_i, t) dt $$</p>
<p>According to my note</p>
<blockquote>
<p>According to the first postulate of special relativity, the action integral $... | g6723 | [
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0.00548030016... |
<p>Today one of my instructor told me that gases cannot have heat transfer thru conduction because molecules are far apart and so it cannot transfer heat,infact the diffusion process of gases transfers the heat.
my thot at this point was,even though molecules are far apart,they transfer momentum to next molecules and w... | g6724 | [
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<p>Consider the symmetry $SU_L(2)\otimes U_Y(1)$. The entries of $SU_L(2)$ doublet will have same U(1)-charge. How can this be shown mathematically?</p> | g6725 | [
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<p>Why the lepton number conservation is connected with the invariance of the lagrangian under global phase (U(1)) transformation of the wave function? How to distinguish global gauge phase and global "leptonic" phase? And how to build the operator of the leptonic number of particles if there isn't "local leptonic inva... | g6726 | [
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0.053696... |
<p>In my opinion, the <a href="http://en.wikipedia.org/wiki/Grassmann_number">Grassmann number</a> "apparatus" is one of the least intuitive things in modern physics. </p>
<p>I remember that it took a lot of effort when I was studying this. The problem was not in the algebraic manipulations themselves -- it was rather... | g6727 | [
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<p>I am a computer scientist interested in network theory. I have come across the Bose-Einstein Condensate (BEC) because of its connections to complex networks. What I know about condensation is the state changes in water; liquid-gas. Reading the wikipedia articles on the subject is difficult, not because of the maths,... | g6728 | [
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<p>When talking about neutron cross-sections, literature is usually investigating isolated cases of Neutron + Atom. Here, the abundance of hydrogen is dominating neutron fluxes through material.</p>
<p>I wonder whether the reflection or capture propability of neutron radiation changes when the flux is penetrating a sy... | g6729 | [
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... |
<p>So recently in physics class, we learned about the magnetism <a href="http://physicsed.buffalostate.edu/SeatExpts/resource/rhr/rhr.htm">right hand rules</a>. </p>
<p>One of them states that the index finger points in the direction of the velocity of a particle, the middle finger points in the direction of the magne... | g6730 | [
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<p>I have tried to find in the literature a proper nice and beautiful Bloch sphere to describe the trajectory of a nuclear spin, starting in z-axis, using a pulse sequence of an initial 90º pulse with zero phase (which means that the pulse is launched in the x-axis), wait for a time $\tau$, and then launch another 90º ... | g6731 | [
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<p>I'm trying to explain in simple terms what the weak interaction does, but I'm having trouble since it doesn't resemble other forces he's familiar with and I haven't been able to come up (or find on the web) with a good, simple visualization for it. </p> | g6732 | [
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<p>What experiment would disprove <a href="http://en.wikipedia.org/wiki/Friedmann_equations" rel="nofollow">Friedmann model</a> of cosmology?</p>
<p>As a layman, I have read a lot of articles and threads in specialized forums. I am probably wrong, but I developed an impression that that theory is circular, 99.9% self-... | g6733 | [
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0.02801... |
<p>So I have a graph:</p>
<p><img src="http://i.stack.imgur.com/BL8Xn.png" alt="enter image description here"></p>
<p>The value of the gradient/slope is $1.6±0.4$ and the value of the intercept is $0.9±0.4$. But what are the units of the graph? Is the unit of the gradient $v^2M^{-1}$? What about the unit of the inter... | g6734 | [
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<p>I saw a pretty simple homework question <a href="http://physics.stackexchange.com/questions/39281/needed-energy-for-lifting-200-kg-weight">here</a> that asked how much work it takes to lift a 200 kg weight, and while the math for a basic answer is simple the weightlifter in me instead wanted to actually approach thi... | g6735 | [
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<p>Why do cosmic bodies such as planets, stars, satellites revolve?<br>
What made them to revolve after the formation of universe?</p> | g231 | [
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<blockquote>
<p>A uniform rod of mass 1.2 kg and length 1.8 m is pivoted in the horizontal position as shown (black point).<img src="http://i.stack.imgur.com/cGmYy.png" alt="enter image description here">
The rod is at rest and then released. The acceleration due to gravity is $g = 9.8 m/s^2$.
1) Find the rotatio... | g6736 | [
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<p>A big argument by the nitrogen-in-the-tire crowd is that:</p>
<blockquote>
<p><em>Nitrogen atoms are bigger and thus less likely to escape the tire, bringing stability to your tire pressure.</em></p>
</blockquote>
<p>If <a href="http://en.wikipedia.org/wiki/Earth%27s_atmosphere#Composition">Earth's atmosphere is... | g6737 | [
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<p>Analogy: assume that I have constant rain fall and I have a water bucket to collect this rain. If I am rest relative to the earth, I will catch a certain amount of rain. However, if I now move towards the rain, I will increase the amount of water I collect. Now I want to apply this idea to a source emitting photons ... | g6738 | [
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<p>In classical theory, for electrons or protons, charge and angular momentum combine to give the magnetic moment.</p>
<p>Does a similar consequence hold for the generalization of charge to other forces, like the strong force or weak force?</p> | g6739 | [
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<blockquote>
<p><em>If a ball under the influence of gravity falls straight down from a height $h$, collides elastically with the floor, at the instant of collision, what forces does it experience?</em> </p>
</blockquote>
<p>Shouldn't the floor exert a force equal to the weight of the ball, on the ball itself? Then ... | g6740 | [
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<p>Please help me out, I’m missing something.</p>
<p>We know that, right now, space is expanding at roughly 73km/s/Mpc.</p>
<p>This means: two points in space 1Mpc away from each other “move” 73 km farther away every second. Of course they are not actually moving, it’s the space between them that’s expanding: that’s ... | g6741 | [
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<p>An air bubble was initially released from the bottom of an ocean. Assume that the air in the air bubble is an ideal gas and its <strong>temperature remains constant</strong> at 25 degrees Celsius. Is heat added or removed from the air bubble as the bubble rises?</p>
<p>My reasoning is that since total internal ene... | g6742 | [
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<p>I have a theory I think might explain why the universe has accelerated in a way that doesn't require dark energy. I'm wondering if someone has proposed this theory before (did some research and couldn't find anything).</p>
<p>The theory is that the big bang was essentially a huge super-nova-like eruption inside an ... | g6743 | [
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<p>Reference request concerning the <a href="http://en.wikipedia.org/wiki/Hollow_Earth#Concave_hollow_Earths" rel="nofollow">Hollow concave Earth hypothesis</a>. I am searching for this paper: </p>
<blockquote>
<p><em>A Geocosmos: Mapping Outer Space Into a Hollow Earth</em> authored by M. Abdelkader and published i... | g6744 | [
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... |
<p>Is there such as thing as the viscosity of stars in a galaxy, along the lines of gravitational attraction between stars changing the dynamics. </p>
<p>If so, how is that put in terms of the Virial Theorem?</p> | g6745 | [
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<p>Just hoping for some clarity regarding Hamilton's characteristic function (W). When we take a time independent Hamiltonian we can separate the Principle function (S) up into the characteristic function minus $ht$, yes I know its the Legendre transform but,</p>
<p>\begin{equation}
W=S+ht
\end{equation}</p>
<p>Meir... | g6746 | [
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<p>I have been looking at taking the continuum limit for a linear elastic rod of length $l$ modeled by a series of masses each of mass $m$ connected via massless springs of spring constant $k$. The distance between each mass is $\Delta x$ which we use to express the total length as $l=(n+1)\Delta x$. The displacement f... | g6747 | [
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<p>At first, I know the question sounds ambiguous and maybe pseudo-scientific, but it's a thing I've been arguing about with my colleage for quite some time and while neither of us knows much about quantum mechanics besides the popular statements you see in internet, it has been bugging me to finally end the argument.<... | g6748 | [
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<p>Is there a formula that can get sunrise at a particular latitude and longitude (and elevation from sea level)? If so, what it is? (Why does it work?)</p> | g6749 | [
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<p>In standard cosmology models (Friedmann equations which your favorite choice of DM and DE), there exists a frame in which the total momenta of any sufficiently large sphere, centered at any point in space, will sum to 0 [1] (this is the reference frame in which the CMB anisotropies are minimal). Is this not a form o... | g6750 | [
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