question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
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<p>My question was inspired by <a href="https://physics.stackexchange.com/questions/108526/does-the-mass-of-a-star-change-as-it-collapses-into-a-black-hole#108529">this question</a>, which got me thinking. According to <a href="http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation" rel="nofollow">Newton'... | g10194 | [
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<p>Is the following correct for the distance $d$ from the origin $(0,0)$ to point $(t,x)$ in the 2-dimensional
<a href="http://en.wikipedia.org/wiki/De_Sitter_space" rel="nofollow">de-Sitter</a> and <a href="http://en.wikipedia.org/wiki/Anti_de_Sitter_space" rel="nofollow">anti de-Sitter</a> spaces? Here, $t$ is time a... | g10195 | [
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<p>Breit Wigner Formula describes the cross section for interactions that proceed dominantly via a intermediate particle (O*) A+B → O* → C + D:</p>
<p>$$σ = \frac{2\Pi}{k^{2}}\frac{Γ_{i}Γ_{f}}{(E-E_{o})^{2} + (Γ/2)^{2}}$$</p>
<p>A short question: Does the formula apply to situations when the intermediate particle is ... | g10196 | [
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<p>Is there a way in which one can use the BCH relation to find the equivalent angle and the axis for two rotations? I am aware that one can do it in a precise way using Euler Angles but I was wondering whether we can use just the algebra of the rotation group to perform the same computation?</p> | g10197 | [
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<p>In classical mechanics for two mass particles $a$,$b$ we assume the symmetric potential arising from $F_{ab}$ and $F_{ab}$ given by $$U_{ab}(r)=-\int^{r}_{r_{0}}F_{ab}(r')dr'$$ and $$U_{ba}(r)=-\int^{r}_{r_{0}}F_{ba}(r')dr'$$ </p>
<p>The book <strong>mechanics</strong> by <em>Florian Scheck</em> gives $$\frac{d}{dt... | g10198 | [
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<p>On the <a href="https://en.wikipedia.org/wiki/Bohr_radius" rel="nofollow">Wikipedia</a> I found that the Bohr radius is equal to: </p>
<p>\begin{align}
\boxed{r_b=\dfrac{4\pi\varepsilon_0\hbar^2}{m_e{e}^2}}
\end{align}</p>
<p>but while we have been learning Bohr's model we derived the equation for a radius of the ... | g10199 | [
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<p>I am now working on a slab system, but encountered some problems on the classical Hartree potential. This slab system is infinity along x-y plane, and has finite size along $z$ axis $z\in[0,L]$. I found a paper [PHYSICAL REVIEW B 80, 235101 (2009)], in which the Hartree potential reads:</p>
<p>$$V_H=-2 \pi e^2\int_... | g10200 | [
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<p>I am confused between these two quantities: <strong>Isentropic pressure</strong> and <strong>Total pressure</strong>.
Are they the same?</p> | g10201 | [
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<blockquote>
<p><em>A particle of mass $m$, moves around a central force whose potential is $$V(r) = kmr^3 \; (k>0),$$
then<br>
$(c)$ if the particle is slightly disturbed from the circular motion, what is the period of small radial oscillation about $r=a$?</em></p>
</blockquote>
<p>I have the solution but I... | g10202 | [
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<p>I'm very primitive with my thought. So please help if you can in layman terms with an answer to this question.
Here we go- by using a street power pole as the source responder, is it possible to read a speed of a traveling car by using two reflectors on the road which sonar back to the source? My thought is reflect... | g10203 | [
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<p>Let's say you have a short pulse of light which expands radially from a lightbulb, and it impinges upon a mirror and reflects towards a photodetector which you have places somewhere above the mirror.</p>
<p>If the light was monochromatic, then you could easily calculate the Fresnel zone and claim that there are int... | g10204 | [
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<p>I am deriving the results of renormalization for nonlinear sigma model using Polyakov approach. I am closely following chapter 2 of Polyakov's book--- ``Gauge fields and strings''. </p>
<p>Action for the nonlinear sigma model (NLSM) is
\begin{equation}
S= \frac{1}{2e_0^2}\int d^2x (\partial_\mu \mathbf{n})^2.
\e... | g10205 | [
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<p>Related: <a href="http://physics.stackexchange.com/questions/59211/faradays-law-in-a-ring">Faraday's law in a ring</a></p>
<p>The ring launcher is a standard introductory physics demonstration that I assume almost everyone has seen (if not, YouTube it). The explanation of why the ring is launched is explained on ma... | g10206 | [
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<p>If magnetic energy depends on the electron poles within two-fields within a permanent magnets void, how do invidiual atoms react within the attraction or repulsion of poles, and what incurrence does this live by in regards to molecular struction?</p> | g10207 | [
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<p>My book doesn't explain well how to build a doublet of antiparticles that transforms the same way the particle doublet $(p,n)^T$ (proton neutron) does. </p>
<p>They claim $$\tag 1 \vert I=1,I_3=1\rangle = -p\bar n$$ for a composite nucleon-antinucleon system. </p>
<p><strong>Why is $(1)$ true?</strong></p>
<p>Pe... | g10208 | [
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<p>In the following research paper (link provided) , I am not able to figure out how do we get the equation (7) . Please help me out. Also , please recommend the necessary pre requisite text to understand how to reach equation (7). I am able to figure out the equation (5) (inside the quantum dot - using Bessel Equation... | g10209 | [
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<p>On page 410 of Griffiths QM 2nd Ed. book, he begins an analysis to evaluate the integral:
$$\frac{1}{2i}\int_{-\infty}^\infty \frac{s \sin{(sr)}}{(s-k)(s+k)}\mathrm{d}s.$$
To exploit Cauchy's formula, he dissolves the integral into:
$$\int_{-\infty}^\infty \frac{s e^{isr}}{(s-k)(s+k)}\mathrm{d}s-\int_{-\infty}^\inft... | g10210 | [
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<p>As described in <a href="http://arxiv.org/abs/1104.4543" rel="nofollow">"A class of elementary particle models without any adjustable real parameters"</a>, <a href="http://arxiv.org/abs/1011.0061" rel="nofollow">"The Conformal Constraint in Canonical Quantum Gravity"</a>, and <a href="http://arxiv.org/abs/1009.0669"... | g10211 | [
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<p>In the literature on topological insulators and superconductors the 'bulk-boundary correspondence' features quite heavily. One version of this conjecture says roughly: "At an interface between two materials belonging to the same symmetry class with bulk invariants n and m, precisely |n-m| gapless edge modes will app... | g10212 | [
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<p>Assume we know two different mixed states, p and q, and an efficient (quantum) algorithm for creating such two. Does it follow that there exists a computationally efficient method/measurement for optimally(that is, according to the trace norm distance) distinguishing between the two? </p> | g10213 | [
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<p><strong><em>Steady flow</em></strong> is the condition in which the flow velocity profile does not vary with time.</p>
<p>Mathematically this is translated to $\frac{\partial \mathbf{v}}{\partial t} = 0$, for example in the derivation of Bernoulli's principle for the Navier-Stokes equation.</p>
<p>Why is it not $\... | g10214 | [
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<p>If the Universe did start from a single point, then wouldn't all particles be fundamentally entangled? How then could there be a truly "pure" state?</p> | g10215 | [
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<p><a href="http://en.wikipedia.org/wiki/Plasma_%28physics%29" rel="nofollow">Plasma</a> is the fourth state of matter. Wikipedia says, that all the gaseous atoms will be ionized into positive ions and free electrons at extremely high temperatures. But, what does this explain in a <a href="http://en.wikipedia.org/wiki/... | g10216 | [
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<p>To ask a more specific one for the rotation curves of elliptical galaxies, and hope from there to later understand the dynamics of spiral galaxies. </p>
<ol>
<li><p><strong>Treating the galaxy as an isothermal gravitational gas sphere, what is the equation of Density for an elliptical galaxy?</strong> </p></li>
<li... | g10217 | [
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<p>Consider an object orbiting around a point with radius $r$ and angular velocity $\omega$. Here its linear velocity is $v=\omega r$. If we choose a large enough $r$ and reasonable $\omega$, $v$ might be greater than $c$.</p>
<p>If this fact is impossible, what prevents it from happening?</p> | g121 | [
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<p>What are some molecules stable in outer space that are unstable under terrestrial conditions?</p>
<p>So there are many molecules that violently react on Earth because they're too charged, have unfilled valence electrons, or have extra electrons in their outer shells. But in space, they often can't really "find" ano... | g10218 | [
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<p>We can see the definition of quantum anomaly in terms of Lagrangian path integral formulation. What is the definition of quantum anomaly in terms of Hamiltonian operator approach or even more directly in terms of wave functions? What are the characteristics of anomalous Hamiltonians or wave functions?</p> | g10219 | [
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<p>The sun is made of fire but fire needs oxygen right? So.. </p>
<ol>
<li><p>Why can there be flames in space, while there's no oxygen? </p></li>
<li><p>Same idea as with the rocket engines of the spaceship, which also produce fire while there's no oxygen?</p></li>
</ol> | g10220 | [
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<p>I don't understand this sentence (emphasis added):</p>
<blockquote>
<p>A consideration of Bragg's law (nλ = 2dsinθ), i.e. the relationship between scattering angle (θ) and the interplanar spacing (d) shows that <strong>if the wavelength (λ) is increased the total diffracted intensity becomes less sensitive to the... | g10221 | [
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<p>If the photons of a laser would produce a radiation pressure upon whatever it shown upon wouldn't it be accurate to say that the laser would be propelled in the opposite direction of its beam?</p> | g10222 | [
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<p>Of course aerodynamics factors into this question, and the faster you are moving the more air you have to push out of your way, the more energy you use. But would the difference be only a small percentage change or would it be a lot more than that. Essentially would a certain unit of energy be able to move a certain... | g10223 | [
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<p>Today, we were being given a lesson on Thermodynamics when my brain encountered an error.</p>
<p>It is easy to derive that, $\Delta U$, the internal energy change for constant volume process is given by,
$$
\Delta U = nC_V \Delta T
$$
and that enthalpy change for a constant pressure process is given by
$$
\Delta H ... | g10224 | [
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<p>(At the suggestion of the user <em>markovchain</em>, I have decided to take a very large edit/addition to the <a href="http://physics.stackexchange.com/questions/52211/is-the-quantization-of-gravity-necessary-for-a-quantum-theory-of-gravity#comment111930_52211">original question</a>, and ask it as a separate questio... | g10225 | [
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<p>1.In classical mechanics, using Newton's laws, the ellipticity of orbits is derived. It is also said that the center of mass is at one of the foci. </p>
<p>2.Each body will orbit the center of the mass of the system.</p>
<p>My question is : Are the assumptions in 1 and 2 correct? </p>
<p>Follow up question : Assu... | g856 | [
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<p>I am aware that the field in General Relativity (the metric, $g_{\mu\nu}$) is not completely physical, as two metrics which are related by a diffeomorphism (~ a change in coordinates) are physically equivalent. This is similar to the fact that the vector potential in electromagnetism ($A_\mu$) is not physical. In el... | g10226 | [
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<p>Does a sound at 50dB at 1m have the same intensity of a sound of 51dB at 10m, and also the same intensity of a 52dB sound at 100m?</p> | g10227 | [
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<p>Is it possible for a liquid to exist in a high quality vacuume? For example, a few Torr.</p>
<p>If so what are the methods for doing this?</p> | g10228 | [
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<p>What is <a href="http://www.google.com/search?as_q=thermal+undulation" rel="nofollow">thermal undulation</a> in the context of lipid bilayers? Is it another word for "thermal fluctuation"?</p> | g10229 | [
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<p>According <a href="http://gamedevelopment.tutsplus.com/tutorials/custom-2d-physics-engine-oriented-rigid-bodies--gamedev-8032" rel="nofollow">this tutorial</a>, formula number 5:</p>
<p>$$j = \frac{-(1 + e)((V^{A} - V^{B}) * t)}{\frac{1}{mass^{A}} + \frac{1}{mass^{B}}}$$</p>
<p>translates into formula number 6:</p... | g10230 | [
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<p>How does <a href="http://en.wikipedia.org/wiki/Divergence_theorem" rel="nofollow">divergence theorem</a> holds good for electric field.
How does this hold true- </p>
<p>$$\iiint\limits_{\mathcal{V}} (\vec{\nabla}\cdot\vec{E})\ \mbox{d}V=\mathop{{\int\!\!\!\!\!\int}\mkern-21mu \bigcirc}\limits_{\mathcal{S}} {}... | g326 | [
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<p>Can somebody explain the proof of Gauss's theorem / <a href="http://en.wikipedia.org/wiki/Divergence_theorem" rel="nofollow">divergence theorem</a> taking the vector as electric field</p>
<p>$$\iiint(\nabla\cdot\vec E)\mbox{ d} V=\iint \vec E \cdot\hat{n} \mbox{ d} A?$$</p> | g326 | [
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<p>How would you prove Gauss' law for an asymmetrical closed surface? I can find it for symmetrical surface but couldn't for Asymmetrical surfaces.</p> | g327 | [
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<p>I encountered the following question in a previous year paper of a graduation-based test.</p>
<blockquote>
<p>On a certain night the moon in its waning phase was a half-moon. At midnight the moon will be (choose one of the following)</p>
<ol>
<li>on the eastern horizon. </li>
<li>at 45 degrees angular h... | g10231 | [
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<p>I'm dealing with angular momentum, or particularly spin, on my quantum mechanics course; I guess the Pauli matrices thing is a more general one, but I'd like to illustrate my doubt with them (maybe get a deeper answer). Say, for the electron ($s=1/2$), why does
$$S_z=\begin{pmatrix}\left\langle\frac{1}{2},\frac{1}{2... | g10232 | [
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<p>I have used FEA for a couple of years now, but using it and using it correctly are two different things, safety factor is not the solution to everything. I have the feeling I won't be using it right unless I have a clear answer to that question:</p>
<p>I am aware elements must be close to their ideal shape (based o... | g10233 | [
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<p>In a car which phenomenon, diffraction or the resonant frequency of the car, lends itself more to the ability of bass to go farther?</p>
<p>Related Answer: <a href="http://physics.stackexchange.com/questions/18090/why-do-bass-tones-travel-through-walls">Why do bass tones travel through walls?</a></p> | g10234 | [
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<p>''electric field always undergoes a discontinuity when you cross a surface charge $\sigma$'' GRIFFITHS</p>
<p>In the derivation; Suppose we draw a wafer-thin Gaussian Pillbox, extendind just barely over the edge in each direction. Gauss law states that:</p>
<p>$\int_{S} E \cdot A = Q_{enc}/ \epsilon $</p>
<p>and ... | g10235 | [
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<p>Concerning combustion of fuel droplets:</p>
<p>Why is the mass fraction of fuel on a fuel droplet surface slightly less than one?</p>
<p>It is known that the temperature is below the boiling point at the fuel droplet surface, so there is no evaporation at the surface and therefore my intuition tell me that the mas... | g10236 | [
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<p>I just started to read the book 'A Brief History of Time' by Stephen Hawking. Actually When he was talking of the idea of infinite density 'thing' before big bang suddenly the mathematical function came to my mind was delta Dirac impulse function I am familiar at engineering classes. But that was normally either in ... | g10237 | [
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<p>In connection with a <a href="http://scifi.stackexchange.com/questions/15825/in-back-to-the-future-why-the-energy-to-travel-in-time-was-1-21-gwatts">related question on Science Fiction & Fantasy</a> Stack Exchange page: does this number 1.21 GW make any sense? What is 1.21 GWatts? With what can you compare it? C... | g10238 | [
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<p>I'm interested in building a robot with a carbon shell, and was wondering where I could find some ressources about how to work with carbon. For example, where to find it, how to shape it, and how to work with it.</p>
<p>Thank you</p> | g10239 | [
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<p>Let's say we try to remove the event horizon of a Kerr black hole by throwing in matter with some large angular momentum. If it starts with GM > a, could we increase a at all? Would such a particle be able to enter the black hole?</p> | g10240 | [
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<p>Reading some books there is something that i don't understand. Is the capacitors topics when they name a potential difference between conductors.
As i know, Potential difference is the difference in electric potential energy per unit charge between two points.</p>
<p>So, talking about capacitors, the potential diff... | g10241 | [
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<p>We know that Quantum Theory should be considered as a framework in which all other theories/forces (Strong, Weak, EM and Gravity) exist.</p>
<p>For example, we have the Quantum Chromodynamics, Quantum Flavordynamics (Electroweak), Quantum Electrodynamics (but still no Quantum GR).</p>
<p>When I think about this, i... | g10242 | [
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<p>When an object is submerged in a fluid (e.g., water), there is a pressure on the object due to weight of water by p=mgh, but why is there an upward buoyant force?</p> | g10243 | [
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<p>If one would like to study <a href="http://en.wikipedia.org/wiki/CP_violation" rel="nofollow">CP violation</a>, what would be the prerequisites for it? </p>
<p>For example, until now I have not studied quantum field theory and have done very little classical field theory, but I have done some quantum mechanics&mdas... | g10244 | [
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<p>Already the early papers on supergravity were written using computer algebra software to do some calculations. What modern packages do people normally use for doing such calculations? Of course Mathematica and presumable a lot of other programs can be made to do such calculations, I am wondering if there are special... | g10245 | [
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<p>With regards to the equations: $P=VI$ ; $P=V^2/R$ and $P=I^2R$, if you are given $P$, $V$, $I$ and $R$ for a circuit, how do you know which equation to use?</p>
<p>Does the use of an equation have to do with whether the circuit is in series or in parallel?</p> | g10246 | [
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<p>Say we are given the scattering cross section for neutrinos from $d$ and $\bar{u}$ quarks as $\frac{d\sigma^{d}}{dQ^2}=\frac{G_F^2}{\pi}$, $\frac{d\sigma^{\bar{u}}}{dQ^2}=\frac{G_F^2}{\pi} (1-y)^2$, and the $u,\bar{d}$ cross sections are zero, how is the <strong>average nucleon cross section</strong> determined?</p>... | g10247 | [
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<p>Im trying to understand this line given by our prof. :<br/>
"Representing fluid parameters as a function of the spatial coordinates($x$, $y$, $z$) and time $t$.</p>
<p>For example:
$$\vec{V} = u(x,y,z,t) \vec{i} + v(x,y,z,t) \vec{j} + w(x,y,z,t)\vec{k}$$</p>
<p>NOTE: $\vec{i}$, $\vec{j}$ and $\vec{k}$ are the unit... | g10248 | [
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<p>After the data from the cosmic microwave background has been collected by WMAP or Planck, what types of analysis is needed to conduct in order to deduce the cold dark matter density and the distribution of matter in the universe? In other words - How the CMB anisotropy measurements giving us evidence for the existen... | g10249 | [
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<p>In classical mechanics, a center of <em>mechanical</em> momentum frame can always be found for a system of particles interacting with one another locally. For an electromagnetic system where the charges interact non-locally via the electromagnetic field, a center of <em>total</em> momentum can likewise always be fou... | g10250 | [
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<p>I've recently encountered a path integral of the form</p>
<p>$$\int \delta[a\phi+b\phi']\,L(\phi,\phi')\;\mathcal D\phi\mathcal D\phi'$$</p>
<p>(where $a$, $b$ are integers) and would like to eliminate one of the $\mathcal D\phi$ integrations.</p>
<p>Is this possible? A change of variables doesn't seem to work si... | g10251 | [
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<p>Absence of magnetic charges is reflected in one of Maxwell's fundamental equations:
$$\operatorname{div} \vec B = 0 \text{ (1).}$$
This equation allows us to introducte concept of vector potential:
$$\vec B = \operatorname{rot} \vec A.$$
Using this concept, it is possible to express Maxwell's equations in a gracefu... | g10252 | [
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<p>I am going over Coulomb's law and there is something that is a bit
confusing for me:</p>
<p>According to Coulomb's law, if I have a charge $q_{1}$ at a point
$\vec{r_{1}}$ and a charge $q_{2}$ at a point $\vec{r_{2}}$ then
the force that the first charge applies to the second charge is given
by
$$
F_{1,2}=\frac{Kq... | g10253 | [
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<p>I'm analyzing the experimental data obtained during Frank-Hertz experiment (conducted with Hg atoms):
<img src="http://i.stack.imgur.com/D3K82.jpg" alt="enter image description here"></p>
<p>Accelerating voltage values were multiplied by 0.1 during measurement (i.e. the mean value of energies differences if not 0.5... | g10254 | [
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<p>I have a pretty simple homework question, but I can't rap my head around it. </p>
<p>In the question a swimmer of $55 \mbox{ } \mathrm{kg}$, jumps off a stationary raft of $210\mbox{ }\mathrm{kg} $. The swimmer jumps off the raft with a speed of $4.6 \mbox{ } \mathrm{ms}^{-1} $. ... | g10255 | [
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<p>When I turn my microwave oven over the stove on, it will cause a motion sensor light in the hallway next to the kitchen to got off and on. This affect can be reproduced anytime. I did notice that we stand in front of the microwave instead of out of the room, the motion sensor light goes on then off then on repeat... | g10256 | [
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<p>When I was in the university (in the late 90s, circa 1995) I was told there had been research investigating the $2$ (the square of distance) in the Newton's law of universal gravitation.</p>
<p>$F=G\frac{m_1m_2}{r^2}$</p>
<p>Maybe a model like </p>
<p>$F=G\frac{m_1m_2}{r^a}$</p>
<p>with $a$ slightly different fr... | g466 | [
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... |
<p>Suppose I have a spherically symmetric potential and I can find its cross section in configuration space (i.e position-space), $d\sigma / d\theta$. Now I need to find its distribution $d^2\sigma / p_rdp_rdp_\theta$ in momentum space. How can I do this? </p> | g10257 | [
0.03547104075551033,
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<p>I am trying to desgin a little penny balista toy (built with a 3d printer).</p>
<p>The short description is that there is a track with a mass ("hammer") that slides along it. There is a stack of pennies in a hopper at the front. The hammer goes from back to the front, slams into the penny at the end and launches it... | g10258 | [
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<p>I just read <a href="http://physics.stackexchange.com/questions/9584/stability-of-hypothetical-lunar-atmosphere">stability of hypothetical lunar atmosphere</a>. From the correct answer, i understand, the low escape velocity from Luna is part of the reason it is unable to retain an atmosphere.</p>
<p>Titan apparentl... | g10259 | [
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<p>Are photovoltaic cell arrays on a satellite the same that are used within Earth, or is there some difference in their construction given the differing environment in which they are to operate? Does the efficiency/life of a photovoltaic cell change depending upon whether it is within/without Earth's atmosphere? </p> | g10260 | [
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<p>I am writing a physics engine to map the rotary and translatory movements of a uniformly dense solid cylinder within 3d space. If a vectored thrust is applied to one end of the cylinder at an arbitrary angle (say 30 degrees off center), I understand this will cause both a rotation and translation of the body.
ROTATI... | g10261 | [
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<p>An object has an initial velocity of <strong>v</strong> and should stop in <strong>d</strong> meters, in <strong>t</strong> seconds where <strong>v*t >= d</strong>. I can predict that this acceleration won't be constant but it is a function of time and this problem may not have one exact solution for given <strong>v... | g10262 | [
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<p>About how fast can a small fish swim before experiencing turbulent flow around its body? An Engineering Problem! Please go through this question step by step. :D</p> | g10263 | [
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<p>I'm a high school student. I still don't understand what <a href="http://en.wikipedia.org/wiki/Turbulence" rel="nofollow">turbulence</a> is. Please can you explain what it really is? This is what I think it is: rotating motion of water when a particle travels at a velocity of $V$. </p> | g10264 | [
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<p>The following is the equation which, I want to know, if it is valid in relativistic domain. Consider two equal charges moving in same direction with velocity $v$ and charge $q$ at a separation of $d$. The magnetic force acting between them is $(1/4\pi\epsilon_0 c^2)v^2q^2/d^2$ and the electrostatic force is $(1/4\p... | g10265 | [
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<p>my physics teacher told me about the refraction and its applications one of them was<br>
<strong>2 minutes of early sunrise</strong> and after she explained this effect she concluded that days are<br>
<strong>2 minutes longer</strong> than one would naively presume. </p>
<p>However, I think that <strong>her conclu... | g10266 | [
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... |
<p>I have an extremely ridiculous doubt that has been bothering me, since I started learning quantum mechanics. </p>
<p>If we consider the finite dimensional vector space for the spin$\frac{1}{2}$
particles, I guess it is nothing but $\mathbb{C}^2$. Each vector has two components (which is why it is two dimensional r... | g10267 | [
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<p>I know that it is a very old question but still I don't find any satisfactory solution for <a href="http://en.wikipedia.org/wiki/Zeno%27s_paradoxes#Achilles_and_the_tortoise">Achilles Paradox</a>. Please explain me the fundamentals of Achilles paradox in terms of stage wise distance covered. Note that it is easily s... | g985 | [
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<p>If I have a vector A=4i+3j and B=5i-2j, how can I find the vector product AxB? I know that given the angle, its C=AB sin theta, but how can I solve this without the angle?</p> | g10268 | [
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<p>Considering the typical situation of a <a href="http://www.youtube.com/watch?v=8H98BgRzpOM" rel="nofollow">rotating bicycle wheel held by one end of its axle by a rope tied to the ceiling</a>: gravity torque is the time derivative of the angular momentum, and in this case is perpendicular to the angular momentum vec... | g10269 | [
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<p>Again, as usual Schwinger leaves me startled as he writes, the Hermitian displacement operator in 2D is
$$ G = p_1\delta x_1 +p_2 \delta x_2 $$
Now, we know clearly that this group is an Abelian group, therefore $ [p_1,p_2] = 0$. But the author suggests this is equivalent to saying that the commutator generates an... | g10270 | [
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<p>Is there any way to annihilate matter without the use of anti-matter? And vice versa? I mean, for example is it possible to totally convert the mass of a proton into "pure energy" without use an anti-proton?</p> | g10271 | [
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<p>What is <a href="http://en.wikipedia.org/wiki/Drift_velocity" rel="nofollow">drift velocity</a>? And why in some books it is expressed as <strong>drift speed</strong> and not <strong>drift velocity</strong>?Are these different ?</p>
<p>Does it mean that the electron will have extra velocity opposite to the directio... | g10272 | [
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<p>Does anyone know what Feynman was referring to in this interview which appears at the beginning of The Feynman Tips on Physics? Note that he is referring to something that did not appear in the Feynman lectures.</p>
<blockquote>
<p>I didn't like to do the second year, because I didn't think I had
great ideas a... | g10273 | [
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0.0... |
<p>Let us begin in a two-dimensional Euclidean plane. The vector is e.g.
$\vec{V}(x,y)$
It is often useful – but in this case, it's just a mathematical trick that doesn't make the complex numbers "fundamental" – to combine the components into a complex number,
$z=x+iy$.
The two-dimensional rotations in $SO(2)$ are full... | g10274 | [
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0.01744929701089859,
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0.0307602696120739,
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<p>I'm working through some basic theory on periodic potentials, and I would appreciate help in understanding the crystal momentum. Suppose we have a Bravais lattice with lattice vectors $\textbf{R}$. There is an associated reciprocal lattice with lattice vectors $\textbf{K}$ such that $\textbf{K} \cdot \textbf{R} = 2\... | g10275 | [
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0.0018... |
<p>If we live in more than three spatial dimensions, is it not right to conclude that all matter observable to us shares almost the same coordinates of extra dimensions. Or is it just that ordinary interactions (like electromagnetism) are simply independent of these extra dimensions?</p> | g10276 | [
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<p>I currently reading <a href="http://www.damtp.cam.ac.uk/user/tong/string/string.pdf" rel="nofollow">Tong's text on String Theory</a>.</p>
<p>In Chapter 4.1.1 he alludes to a technique to derive conserved currents</p>
<blockquote>
<p>Recall that we can usually derive conserved
currents by promoting the constant... | g10277 | [
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<p>In some supergravities you have the gravition, gravitino, graviphoton and graviscalar. Each is analogous to each other in only sharing gravitational properties and nothing else. They differ by spin in the order of 2, 3/2, 1 and 0. What is the spin 1/2 analogue named?</p> | g10278 | [
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<p>I'm studying the behaviour of superconductors in different cases, and I can't understand this. </p>
<p>We have a superconducting cylinder with a coaxial hole inside (vacuum). When the cylinder is cooled below it's critical temperature, and later a magnetic field is applied (case a), the superconductor will exclude ... | g10279 | [
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0.000... |
<p>Its not about availability of oxygen for combustion of gases. I want to know since there is no air or friction providing things in vaccum, how the force applied by a jet engine get can make thrust to move the vechile</p> | g10280 | [
0.033671215176582336,
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0.03682699054479599,
0.0311... |
<p>I have a copy of David Bohm's <a href="http://rads.stackoverflow.com/amzn/click/0415404258" rel="nofollow">Special Theory of Relativity</a> and also a copy of T.M. Helliwell's <a href="http://rads.stackoverflow.com/amzn/click/1891389610" rel="nofollow">Special Relativity</a>. I was wondering if anyone has used these... | g328 | [
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<p>Consider the experiment in <a href="http://www.kids-fun-science.com/air-pressure-experiments.html">this link</a>.</p>
<p>The experiment includes using a ruler as a lever, with an inflated balloon on one side and a balloon which is not inflated on the other.</p>
<p>The aim of the experiment is to show that air has ... | g10281 | [
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<p>Well, is the basic difference between the work that we learn in <a href="http://en.wikipedia.org/wiki/Work_%28physics%29" rel="nofollow">Mechanics</a> and that in <a href="http://en.wikipedia.org/wiki/Work_%28thermodynamics%29" rel="nofollow">Thermodynamics</a>? This is because in Mechanics, whenever work of magnitu... | g10282 | [
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<p>I am reading the following article:
<a href="http://www.kof.zcu.cz/st/dis/schwarzmeier/galaxy_models.html" rel="nofollow">http://www.kof.zcu.cz/st/dis/schwarzmeier/galaxy_models.html</a>
and am currently at section 5.6 (positions of bodies in a galaxy).</p>
<p>I am trying to redo the simulations myself in Python, b... | g10283 | [
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<p>I have a problem understanding the connection between the accoustic peaks in the CMB spectrum and the baryon oscillation picture. On the one hand it is stated, that the odd accoustic peaks (1,3,5..) are compressions peaks, i.e. peaks which have just reached maximum compression as decoupling occured. On the other han... | g10284 | [
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<p>The symmetry of the standard model between the fermions and each generation of them is strongly suggestive of components as proposed in the <a href="http://en.wikipedia.org/wiki/Rishon_model" rel="nofollow">Rishon model</a>. Having discovered the Higgs boson, we know the Higgs plays a strong role in the mass of the ... | g10285 | [
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... |
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