question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>I am facing a challenge in my project regarding optical alignment.</p>
<p>See the figure:</p>
<p><img src="http://i.stack.imgur.com/q3KTF.png" alt="enter image description here"></p>
<p>The challenge is with the vertical optical system alignment. I considered placing a mirror and check back if the image and sourc... | g18304 | [
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<p>I got really confused about the graph of the relationship between wavelength and intensity of black body radiation. What does the peak stand for? And what does the graph tell us?How can we analyze it?</p> | g566 | [
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<p>Three blocks of masses $M_A=15.0 \; \text{kg}$, $M_B=12.0
\; \text{kg}$, and $M_C=8.0 \; \text{kg}$ sit next to each other on a
frictionless surface. A force $F=54.0 \; \text{N}$ acts on
Block $A$ at an angle of $\theta=25°$ below the horizontal. </p>
<p>Calculate the magnitude and direction of all forces acting... | g18305 | [
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<p>I'm a total novice to physics so please forgive me but I'm looking for a starting point on sound, frequencies and everything to do with electricity. Preferably books because my internet usage is limited, would love to here back so please.. help me understand the world </p> | g98 | [
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<p>Two questions:</p>
<ol>
<li>Assuming you dive head first or fall straight with your legs first, what is the maximal height you can jump into water from and not get hurt?
<br>In other words, an H meter fall into water is equivalent to how many meters concrete-pavement fall, force wise? (I'm assuming the damage cause... | g18306 | [
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<p>I'm not really sure that this is the place to ask this question, but anyway. I'm curious is it possible to create a solar cell (not panel with 2$ cells from ebay) in home. If not why and if yes how. Thanks in in advance.</p> | g18307 | [
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<p>One of the things I never understood, but was too afraid to ask is this: how should I think of things like $\frac{\text{kg}}{\text{s}^2}$. What exactly is a square second? Square foot makes sense to me because I can see it, but square second? I've always assumed it's just one of those things you're supposed to deal ... | g18308 | [
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<p>If one has a deterministic model of physics, why is causality so important?</p>
<p>Let's work in a fixed frame. Suppose that event A in the future causes event B, which happens before event A. Now, given the conditions at some time $t_0$ before both events (or at the time of event B), one can predict what will happ... | g18309 | [
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<p>From what I understand, the <a href="http://en.wikipedia.org/wiki/Uncertainty_principle">uncertainty principle</a> states that there is a fundamental natural limit to how accurately we can measure velocity and momentum at the same time. It's not a limit on equipment but just a natural phenomenon.</p>
<p>However, is... | g139 | [
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<p>Quantum field theory arises from the requirement that the S-matrix is lorentz scalar and obeys the cluster decomposition principle. </p>
<p>I want to know if there are <em>other ways to build invariant S-Matrices</em> that obey the cluster decomposition principle without using quantum field theory. QFT is the most ... | g18310 | [
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<p>$\mathcal N=4$ SYM has an $\mathrm{SL}(2,\mathbb Z)$ duality group. This can be thought of in two ways:
1. This theory can be obtained by compactifying the 6D $\mathcal N=(2,0)$ theory on a torus, and this group comes from the group of large diffeomorphisms of the torus. </p>
<ol>
<li>The theory can also be obta... | g18311 | [
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<p>I am trying to conceptually understand why the angle which produces the greatest range for a projectile launched with an elevation is not 45 degrees. I have exausted all other options, and I hope that someone here can explain it to me conceptually. That is, I am currently not at the mathematical level to explain thi... | g18312 | [
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<p><em><strong>Note:</em></strong> I'm a layman in physics. Excuse me if this question is a duplicate, I lack the knowledge in technical terms to find the answer to this by research (I've tried).</p>
<p>Let's suppose there's a star moving faster than the speed of light (relative to earth) due to the expansion of the u... | g567 | [
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<p>I am researching low temperature, near absolute zero, and in particular Bose Einstein Condensate. There is a lot of research information, but it is confusing, and not explained. Technically a BEC is formed from a Gas or liquid at somewhere below 1K. The only three elements that could still be in a liquid state at &l... | g18313 | [
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<p>Accelerated electrons emit electromagnetic waves, but those at rest do not. So accelerated electrons emit energy and those at rest do not. (At rest hypothetically, I know it is not permissible according to uncertainity principle)</p>
<p>But lets assume that we keep an electron at rest and start accelerating ourselv... | g18314 | [
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<p>Are there any software packages/frameworks for running weather forecast numeric models on a single PC? I've found at least two instances, but I'm interested to know whether there are more and where I may find it.</p>
<p>My preference is for</p>
<ol>
<li>open-source software (but other licensing schemes can be cons... | g18315 | [
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<p>I am doing research with a double slit experiment, using a beam splitter and 2 lengths of fiber optic cable, whose ends brought close together form the effective double slit. I notice that the interference pattern shifts over time and suspect thermal expnsion/contraction to be changing the relative path lengths and ... | g18316 | [
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<p>I am developing a small computer program that involves moderately simple simulation of elliptical Kepler orbits for fictional, generated star systems. I'm doing this without much prior knowledge of orbits (apart from some basic equations) or astrophysics in general.</p>
<p>I'm attempting to create loosely realistic... | g18317 | [
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<p>Imagine a steady state, one-dimensional, compressible flow in a horizontal pipe of constant cross sectional area. This flow can be isothermal, adiabatic (Fanno), or diabatic (Rayleigh). As an example, the relevant macroscopic energy balance for Fanno flow is:
$$\Delta h=-\Delta (KE)$$<br>
In other words, Fanno flow... | g18318 | [
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<p>In 2006, Demokritov et al have reported that they have achieved the observation of quasi-equilibrium magnon Bose-Einstein condensation (BEC) in YIG at finite (room) temperature by using the method called "microwave pumping" <a href="http://www.nature.com/nature/journal/v443/n7110/abs/nature05117.html" rel="nofollow"... | g18319 | [
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<p>Cylindrical compression springs are everywhere and then some applications choose other forms like rectangular or unique polygonal form. What impact does the form of a compression spring have and how do you calculate an optimal solution?</p> | g238 | [
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<p>There are quite a few nagging questions I have been having over the years, I do not require a full explanation, just some guidance in my assumptions and pointers if I am very wrong.</p>
<p>My basic knowledge of a lightbulb, is that the power dissipated by the tungsten filament heats it up to around 2,700–3,300K (in... | g18320 | [
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<p><a href="http://youtu.be/zPS1FRzQH7Y">Here we see a photobombing trick</a>, used by Harley Davidson.</p>
<p>They achieve to incrust a logo on a photo you take. The logo is invisible with naked eyes and only revealed on the photo you take.</p>
<p>How does it work?</p> | g18321 | [
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<p>If I take the Lorentz position transforms between two inertial frames, $S$ and $S'$ moving with relative velocity $u$ along a common axis (say $x-x'$), then:</p>
<p>$$x'=\gamma(x- ut)$$
$$y'=y$$
$$z'=z$$
$$t'=\gamma\left(t-\frac{ux}{c^2}\right)$$</p>
<p>If the particle has velocity $(v_x,v_y,v_z)$ in $S$ then in $... | g18322 | [
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<p>A recent news item in <a href="http://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379" rel="nofollow">Nature</a> promotes Wetterich's preprint <a href="http://arxiv.org/abs/1303.6878/" rel="nofollow">"A Universe without expansion"</a>. All sounds very exciting but hard to judge for non-e... | g18323 | [
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<p>Sorry maybe I ask a very naive and stupid question. The motivation to ask it is as follows:</p>
<p>I am considering the parity protection of Majorana qubit based on Kitaev's 1D model. As we know, in the Kitaev's chain model there are two free MFs on the end of the chain and the information in these two free MFs is ... | g18324 | [
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<p>It is commonly stated that there are four fundamental forces, or interactions, in nature. It is natural to consider which of those is responsible for the normal force we meet in elementary physics. What is the nature of the force exerted on a body by the floor on which it rests?</p>
<p>Of the four forces, the only ... | g268 | [
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<p>You have a photon traveling with E=hf and you trap it between two perfectly reflecting mirrors (like a QM particle in a box). The photon has to make a standing wave between the mirrors and its spacial frequency is dependant on the distance between the mirrors, L. Its time frequency, and hence energy are derived from... | g18325 | [
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<p>I've been trying to wrap my head what happens when a force is exerted on the surface of a swimming pool / pond, or underneath it. Examples include a small boy cannon-balling into the deep end, or a fish flipping its tail under water. I assume the water is contained on all sides except the top.</p>
<p>Pascal's Law... | g18326 | [
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<p>I see in literature very different (and chaotic) descriptions of what is <a href="http://en.wikipedia.org/wiki/Chaos_theory" rel="nofollow"><em>deterministic chaos</em></a>.</p>
<p>Can you explain to me based in a type of <em>formal definition</em>, which <em>principles</em> need to be exactly fulfilled in order to... | g18327 | [
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<p>When you spin an asymmetrically weighted, 2D disk-shaped top, the heavy part actually rises to the top. Why is this?</p>
<ol>
<li><p><a href="http://www.youtube.com/watch?v=h0SZZTBQmEs">http://www.youtube.com/watch?v=h0SZZTBQmEs</a> </p></li>
<li><p><a href="http://www.youtube.com/watch?v=tDr26U49_VA">http://www.yo... | g642 | [
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0.009542855434119701,
-0.010459855198860168,
0.016458069905638695,
-0.00... |
<p>Just what the title states.</p>
<p>Free hydrogen accumulates in the exosphere. At high altitudes there are large volumes of open space between molecules - which would be a hindrance to collection. </p>
<p>Would it be feasible for spacecraft returning to the surface (just thinking aloud) to skim through the exosphe... | g18328 | [
-0.02611706592142582,
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0.0050... |
<p>Suppose I energize one capacitor by connecting it across a battery, allowing it to achieve some potential difference <strong>V0</strong> across its plates, then disconnect it and allow both of its leads to float in air. Next I take a second identical capacitor, whose leads have recently been shorted together, and c... | g18329 | [
0.10389675199985504,
0.027777209877967834,
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0.015554... |
<p>As a layperson who watches way too many theoretical physics and astronomy documentaries, I've seen the folded-piece-of-paper analogy of wormholes a bajillion times. Usually they explain that black holes might connect to other parts of the universe, but they don't really explain what would have to be on the other end... | g18330 | [
-0.023888476192951202,
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0.... |
<p>My understanding of Kolmogorov scales doesn't really go beyond this poem:</p>
<blockquote>
<p>Big whirls have little whirls that feed on their velocity,<br>
and little whirls have lesser whirls and so on to viscosity.</p>
</blockquote>
<p>The smallest scale according to wikipedia* would be
$\eta = (\frac{\nu^3... | g18331 | [
0.012941362336277962,
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0.00531398132443428,
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... |
<p>I dropped a Tic-Tac: </p>
<p><img src="http://i.stack.imgur.com/fM3Ic.jpg" alt="enter image description here"></p>
<p>(no worries, it happened before), and I noticed as it bounced on the floor that it would first jump 20 cm high, and at the next bounce for instance 50 cm high. Shouldn't it always have less energy ... | g18332 | [
0.08784788101911545,
0.05442294478416443,
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0.05727892741560936,
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0.0206817742... |
<p>To find the dimensions of and V0, I must know the dimension of S and e. So I want to know it.</p> | g18333 | [
0.01957673393189907,
0.00804668664932251,
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<p>A lot of physicists would have agreed that General Relativity is a <strong>beautiful</strong> theory. </p>
<p>My question is that, what really makes a physics theory beautiful? Is there any criteria that a theory must fulfilled before it is considered beautiful? Is Quantum Mechanics beautiful in the same sense as t... | g18334 | [
0.04313778132200241,
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0.04471525922417641,
-0.0222... |
<p>After accelerating say a proton, half way, if the alternating electric field was cut, would the proton maintain a perpetual circular motion at the same radius?</p>
<p>I know that any charged particle moving in a magnet field radiates energy, so would the proton spiral towards the center?</p> | g18335 | [
0.06479068100452423,
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<p>I am investigating the radiation hardness of electronic memory circuits (EEPROM). The following measurement has been performed:</p>
<p>Beam set-up:
Irradiation occurred perpendicular to the DUT (device under test). The distance between the DUT and a 60Co source was 16 cm leading to a dose rate of 100 Gy/h. The ave... | g18336 | [
0.0112018883228302,
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0.0419548936188221,
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-0... |
<p>There is such a thing, called "stability bound" on mass of the Higgs boson.
The basic idea (as I understand it) is that we take Higgs self-coupling, and calculate its renormalization running. And it turns out that for certain initial conditions, it becomes negative at some higher scales. So the Higgs potential becom... | g18337 | [
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0.002982291392982006,
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0.020832622423768044,
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0.... |
<p><strong>The fact:</strong>
Gliders have ballast tanks that can be filled with water. The addition of ballast increases weight, and this allows the glider to fly at faster airspeeds while maintaining the same glide ratio. This means that if two gliders P1 and P2, being both of the same model, start at some point A, ... | g18338 | [
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0.0033784585539251566,
0.009253494441509247,
0.0... |
<p>My maybe very naive question is, of what practical importance will the discovery of the Higgs-Mechanism be for our technological advance in the near future?</p> | g392 | [
-0.0035757943987846375,
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0.014... |
<p>Just that . I am not too much concerned about the accuracy of measurements .</p> | g18339 | [
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... |
<p>Im teaching a section of Calculus 2 (integration techniques, arc length, surface area, improper integrals, parametric & polar functions, sequences, and series ) next semester and would like to assign a problem set near the end of the course to each student relevant to his or her major.</p>
<p>It can be expected... | g18340 | [
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0.04287567362189293,
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0.006323597393929958,
0.034889500588178635,
-0.01476479135453701,
-0.0003... |
<p>Would our eyes observe any changes?</p>
<p>What about electronic measurement devices?</p> | g18341 | [
-0.03454592823982239,
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0.010670868679881096,
0.0062920148484408855,
0.020887209102511406,
-0.005856520030647516,
0.016883641481399536,
0.024728165939450264,
0.028830254450440407,
... |
<p>I have a question about entanglement in condensed matter physics. It seems that <a href="http://en.wikipedia.org/wiki/Topological_order">topological order</a> origins from long range entanglement, but what is long range entanglement? It is the same as long range correlation? I am interested in this issue and I am ha... | g18342 | [
0.04146640747785568,
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-0.05006048083305359,
0.010797152295708656,
0.03017888218164444,
0.052233513444662094,
-0.038... |
<p>The matrices present in the Dirac equation must have the following properties:</p>
<ol>
<li>$\{a^j,a^k\}_{ab} = 2\delta^{jk}\delta_{ab}$ <br></li>
<li>$\{a^j,\beta\} = 0$ <br></li>
<li>$(\beta^2)_{ab} = \delta_{ab}$</li>
</ol>
<p>How can one show from this that the trace of $\beta$ must be $0$?</p> | g18343 | [
0.02152581326663494,
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0.1284675896167755,
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0.06125129014253616,
-0.025098014622926712,
0.0453550... |
<p><a href="http://en.wikipedia.org/wiki/Delta_baryon" rel="nofollow">The delta baryons</a> (also called delta resonances) are a family of subatomic hadron particles which have the symbols $\Delta^{++}$, $\Delta^{+}$, $\Delta^{0}$, and $\Delta^{−}$ and electric charges +2, +1, 0 and −1 elementary charge respectively.</... | g18344 | [
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0.04389292374253273,
0.00608013104647398,
-0.01567354053258896,
0.040... |
<p>People usually talk about similar (or maybe not?) things like vorticity or enstrophy in fluid mechanics, but no one talks about angular momentum, why?</p> | g18345 | [
0.09053675085306168,
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-0.0173... |
<p>I was reading this article entitled "<a href="https://medium.com/the-physics-arxiv-blog/98345796bba4" rel="nofollow">“Ballooning” Spiders Use Electrostatic Forces To Generate Lift</a>". What I would like to know is how can a electrostatically charged object generate lift within the atmosphere? </p>
<p>If I have a c... | g18346 | [
0.025455791503190994,
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0.007384296040982008,
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0.025729607790708542,
-0.031... |
<p>I know that the Lagrangian $L$ is defined to be $T-V$, i.e. the difference between kinetic energy and potential energy. Also the Action $S$ is defined to be $\int Ldx$ and from this we can derive Newton's 2nd law of motion.</p>
<ol>
<li><p>If we get Newton's second law out, does it mean that the formulation is corr... | g18347 | [
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-0.... |
<p><a href="http://www.youtube.com/watch?v=287qd4uI7-E&list=TLKck2toHQ5E0Jr3xjm9w2qBzOVIHrfNB-" rel="nofollow">This video</a> lists a number of perpetual motion machines that have been proposed throughout history:</p>
<ul>
<li><a href="http://en.wikipedia.org/wiki/File%3aBoyle%27sSelfFlowingFlask.png" rel="nofollo... | g18348 | [
0.01367933489382267,
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-0.016706930473446846,
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0.0007150632445700467,
... |
<p>I am preparing for an exam and one of the questions I have come across asks:</p>
<blockquote>
<p>Define the electric field $\mathbf{E}$ and the magnetic flux density $\mathbf{B}$, in terms of the force on charges and currents.</p>
</blockquote>
<p>By the Lorentz force law we have:</p>
<p>$$\mathbf{F}=q(\mathbf{... | g18349 | [
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0.03199634701013565,
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0.03953903540968895,
-0.02747414819896221,
-0.003... |
<p>The title says it all.</p>
<p>If I was on a bus at 60 km/h, and I started walking on the bus at a steady pace of 5 km/h, then I'd technically be moving at 65 km/h, right?</p>
<p>So my son posed me an interesting question today: since light travels as fast as anything can go, what if I shined light when moving in a... | g109 | [
0.028018774464726448,
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0.005607359576970339,
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-0.03670165315270424,
0.0056573194451630116,
0.04... |
<p>I'm attempting to compare the sensitivity that can be gained by using flat spiral coils against regular solenoid coils. A nice paper <a href="http://smirc.stanford.edu/papers/JSSC99OCT-mohan.pdf" rel="nofollow">here</a> gives reasonably accurate expressions for the inductance of such coils.</p>
<p>More precisely, f... | g18350 | [
0.014444490894675255,
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-0.028984254226088524,
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0.01676040329039097,
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0.028561897575855255,
0.029068036004900932,
0.04... |
<p>Is it possible to make an insulator have two poles like a magnet?</p>
<p>If yes, how to make it?</p> | g18351 | [
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0.03947105631232262,
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-0.0... |
<p>For a spherical mirror, an object at the mirror's center has an image that is also at the center. Its magnification is $-1$. For a video showing this, see <a href="http://www.youtube.com/watch?v=K_p5UoD6ljg" rel="nofollow">here</a>.</p>
<p>If you stand slightly behind the center of a large spherical mirror and ho... | g18352 | [
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0.004793805535882711,
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0.007916870526969433,
-0.0011492585763335228,
0.03804660961031914,
0.03765789419412613,
0.01... |
<p>I've been reading about scalar fields in the context of general relativity, and I found this page: <a href="https://en.wikipedia.org/wiki/Stress-energy_tensor#Scalar_field" rel="nofollow">https://en.wikipedia.org/wiki/Stress-energy_tensor#Scalar_field</a>. It says that the stress-energy tensor for a scalar field is<... | g18353 | [
0.02473890781402588,
0.04404732584953308,
-0.008559582754969597,
-0.008854988031089306,
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0.019375937059521675,
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-0.018981721252202988,
-0.001848786254413426,
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-0.01918015629053116,... |
<p>I am looking for an approach on how to apply Kirchhoff current / voltage law in the infinitely long diode ladder network. Can anyone help me with this ?</p>
<p>I am looking for 1D differential equation or an implicit equation.<img src="http://i.stack.imgur.com/qMNhd.jpg" alt="enter image description here"></p> | g18354 | [
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0.019419418647885323,... |
<p>Feynman in his famous lectures in NZ, talked about an <a href="https://www.youtube.com/watch?v=kMSgE62S6oo&feature=youtu.be&t=41m52s" rel="nofollow">experiment with a reflection of a light from a "back" mirror with stripes on it</a></p>
<p>I tried to find a video of such an experiment, but with no success.<... | g18355 | [
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0.0712742879986763,
0.034157250076532364,
-0.09229094535112381,
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-0.030116282403469086,
0.02421267144382,
0.06456007063388824,
0.008602039888501167,
0.089017301... |
<p>I'm trying to wrap my head around the Force $\rightarrow$ Voltage analogy between mechanical and electrical systems. I have a mechanical system where masses are connected by springs in series, and constant force is applied to the first mass in the chain. There is no damping anywhere in the system.
I'm trying to cons... | g18356 | [
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... |
<p>Can someone make or refer me to the argument for why $U(1)$ Chern-Simons theory in three dimensions cannot be defined by a lattice action? (Unlike Dijkgraaf-Witten theories, which are defined on the lattice.)</p>
<p>Possibly related: what is the "framing anomaly"?</p> | g18357 | [
0.04729118198156357,
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<p>I dong know much about physics, but I was curious about the red shift effect. I think I have the basic idea of it down in that as an object 1 accelerates from object 2 the light from object 1 will appear more red to object 2 because it stretches the light between them. My question is how fast is the red shift effect... | g18358 | [
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0.0406913235783577,
0.019771452993154526,
-0.... |
<p>I have a few questions about the Lorentz force.</p>
<ol>
<li><p>Is it possible to create a Lorentz force helix in a cloud chamber and if so how would this be achieved?</p></li>
<li><p>How would the pressure inside the helix be measured in comparison to the pressure surrounding the helix?</p></li>
<li><p>Is there a... | g18359 | [
-0.02323024906218052,
0.0028483422938734293,
0.01279185339808464,
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0.02684057131409645,
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0.0030446683522313833,
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0.020120637491345406,
0.014214718714356422,
-... |
<h1>The Question</h1>
<p>Given <a href="http://en.wikipedia.org/wiki/Maxwell%27s_equations" rel="nofollow">Maxwell's equations</a> of the form
\begin{align}
\bar{\nabla}\times \bar{B}
=
\dfrac{4\pi}{c} \bar{J}
+
\partial_0 \bar{E}
\\
\bar{\nabla}\times \bar{E}
=
-\partial_0 \bar{B}
\\
\bar{\nabla} \cdot \bar{B}=0
\\... | g18360 | [
-0.015081747435033321,
0.009925504215061665,
-0.022884227335453033,
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0.06326748430728912,
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0.08940604329109192,
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0.027366047725081444,
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0.00157954974565655,
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... |
<p>Sorry if this is a stupid question, but I have often been told that air conditioners are more efficient than an electric heater, because, the story goes, if you put say 1 kW of electricity in the electric heater, you get 1 kW worth of heat, but if you put 1 kW worth of electricity into the air conditioner, you can g... | g18361 | [
0.05675844848155975,
0.009214797057211399,
0.0007112682797014713,
0.04755718633532524,
0.02381664142012596,
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0.018836677074432373,
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0.034423407167196274,
0.02319076471030712,
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0.03... |
<p>I am looking at the 4-vector treatment of special relativity, but I have had no formal training in Tensor algebra and thus am having difficulty understanding some of the concepts which appear.</p>
<p>One such concept is the notion of contra- and co-variant vectors. <a href="http://en.wikipedia.org/wiki/Covariance_a... | g18362 | [
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<p><img src="http://i.stack.imgur.com/yGSEt.jpg" alt="enter image description here"></p>
<p>Could I say that it is conserved during the time, or instead there is a slight decrease?</p> | g18363 | [
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0.0484... |
<p>I read (extern link to a filehoster, <a href="http://www.4shared.com/file/O7aVkmjm/Astonomie355.html" rel="nofollow">page 163 section 4.8</a>) that a variable celestial object like a pulsar or quasar must be smaller than the distance the light travels in its variation period. </p>
<p>Can someone give me a proof or ... | g18364 | [
0.035792410373687744,
0.03276601433753967,
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0.00... |
<p>A follow-on from <a href="http://astronomy.stackexchange.com/questions/1016/meteorites-from-mars">this question</a>, if meteorites can come from Mars, it seems they could have come from Earth and ended up back here (though I'm guessing that to be less likely). Do we have any evidence of this having happened? Would t... | g18365 | [
0.04396666958928108,
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<p>First, to clarify: I'm not asking if perhaps there's a moon that we haven't found yet. The question is, theoretically, would the earth be able to have another stable moon in addition to the current one? Or, if the orbit couldn't be stable, why not? How large/small of a moon would it be able to have? And how do we kn... | g18366 | [
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... |
<p>I am considering buying a telescope with a large primary mirror that is about seven years old - it has a small spot of corrosion on the mirror, that the owner says is stable.</p>
<p>Should I be concerned about this spot spreading to the whole mirror or getting much larger? </p>
<p>What kind of care or treatment sh... | g18367 | [
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<p>I'm reading Griffith's Intro to Quantum Mechanics, and he mentions that in an infinite potential well, a classical particle would simply bounce back and forth between the two walls indefinitely. He then proceeds to calculate the situation for a quantum particle, and gets a whole bunch of <em>stationary</em> state so... | g18368 | [
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<p>So the <a href="http://en.wikipedia.org/wiki/New_Horizons" rel="nofollow">New Horizons</a> project is sending a probe out to image Pluto and Charon. As they're pretty far away, it will take the New Horizons probe nearly 10 years to get there (in 2015, having been launched in 2006). It even received a 'slingshot' s... | g18369 | [
0.019912566989660263,
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0.007532166317105293,
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<p>Suppose I shine an electromagnetic wave on a two-level system. I need to describe how the system evolves in context of quantum field theory i.e. using a quantized EM field in the problem. The first step would be to write down the interaction Hamiltonian. What would it be?</p> | g18370 | [
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<p>Something I can never understand is that where the cosmic background radiation spreads?</p>
<p>If I know well, the cosmic background radiation is actually the light of the Big Bang. If it happened <em>exactly</em> in the same time, it must have spread into the theoretic center of the universe. Which would mean that... | g18371 | [
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<p>I need some help in designing my robotic car. So its going to have 4 wheels, each driven by a 12-volt motor. It occurs to me that the weight of the chassis itself will exert some pressure on the wheels and squeeze them against the ground, right? Thereby increasing friction. So is there a way to calculate the pressur... | g18372 | [
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0.00349... |
<p>I distinctly remember reading some article claiming some physicists had discovered a time-irreversible, subatomic quantum mechanical interaction. Is my memory just foggy or has this really been found?</p> | g18373 | [
0.05479879304766655,
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0.02980007790029049,
0.004747131373733282,
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-0.... |
<p>If we could reach (nearly) the speed of light with spaceships, could we measure the velocity of the Earth by launching three perpendicular rockets, accelerating them, and measuring how much fuel (energy) they needed to reach their maximum speed? </p>
<p>Alternately, could we measure the velocity of Earth by emitti... | g18374 | [
-0.007343997247517109,
0.04676912724971771,
0.019171394407749176,
0.004011454060673714,
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0.0726647600531578,
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0.057944364845752716,
0.0... |
<p>The definition of <a href="http://en.wikipedia.org/wiki/Enthalpy" rel="nofollow">enthalpy</a> is
$$H=E+PV,$$
is it only valid for (ideal) gas? As this naturally comes from the first law of thermodynamics,
$$dE=dQ+PdV$$
for a enclosed system of gas.</p>
<p>Can we talk about the enthalpy of (perfect) fluid and even... | g18375 | [
-0.010544291697442532,
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0.008335539139807224,
0.00544440932571888,
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0.04442917928099632,
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0.029858002439141273,
0.040158554911613464,
-0.017144640907645226,
0.010965341702103615,
-0... |
<p>Reading the marvellous book "The Membrane Paradigm" I stumbled upon a suggested change of variable that I'm not able to deal with.
Starting with the usual Schwarzschild metric for the spatial 3-geometry</p>
<p>$$
ds^2 = \frac{1}{f(r)} dr^2 + r^2 \left( d\theta^2 + \sin ^2 \theta \ d\phi\right)\ \ \ \ \ \ \ \ \ \ \... | g18376 | [
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-0.... |
<p>A quantum computer with 10 qubits is classically equivalent to $2^{10}$ bits. How is this equivalence worked out?</p>
<p>I understand that a single qubit is a vector in a 2-dimensional hilbert space, whose base we can label as $|0>$ and $|1>$. </p>
<p>So, 10 qubits will require a 20-dimensional hilbert space... | g18377 | [
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0.048... |
<p>I tend to believe that there are two elementary properties in electricity:</p>
<ol>
<li>Electric charge</li>
<li>Coulomb's force</li>
</ol>
<p>I think that I can express any other entity in electricity using just these two (by means of other physical theories e.g. relativity).</p>
<p>Am I correct, or there are pr... | g18378 | [
0.006738915108144283,
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0.00809425301849842,
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0.10081750899553299,
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0.01999361626803875,
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0.02474... |
<p>Let's say you spill 10ml of water on the kitchen counter. It forms a small puddle that would evaporate after a while (assuming room temperature and sane humidity).
Would spreading a large, dry towel over the puddle cause the water to evaporate quicker? What I mean is a towel that's much larger than the diameter of t... | g18379 | [
0.04147595167160034,
0.04246315360069275,
0.02471225894987583,
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0.013468525372445583,
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0.046853482723236084,
0.03858... |
<p>A device emits 0.2 μSv/h of gamma rays. How thick does an aluminum sheet need to be to completely stop radiation from coming out ? What equation is to be used to calculate this ?</p> | g18380 | [
0.049996089190244675,
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0.011197852902114391,
0.011805317364633083,
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0.017375009134411812,
0.11159726232290268,
0.003941632807254791,
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<p>It is said that the Earth and solar system are 4.6 billion years old.
Presumably this date is achieved from radioactive decay.
If this is the case, since most of the radioactive elements would have, to have been in existence prior to this time, how is this date established?
I'm sure I am missing the point here, some... | g18381 | [
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0.035785023123025894,
0.07130839675664902,
-0.0085... |
<p>What were the influences for this to occur? Did some things play a bigger part that others?</p> | g18382 | [
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0.04231605678796768,
0.038846... |
<p>I read a line today and don't get it: "Molecules with mirror symmetry like oxygen, nitrogen, carbon dioxide, and carbon tetrachloride have no permanent dipole moments."</p>
<p><a href="http://hyperphysics.phy-astr.gsu.edu/hbase/electric/diph2o.html" rel="nofollow">http://hyperphysics.phy-astr.gsu.edu/hbase/electric... | g18383 | [
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-... |
<p>Background: In the book by Altland and Simons, Condensed matter field theory, in exercise 4.5.7, one is supposed to use the effective field theory method to integrate out the phonon field in an interacting electron-phonon system and find an attractive electron-electron interaction. </p>
<p>The form found for the ef... | g18384 | [
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0.015325749292969704,
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0.05728170648217201,
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0.... |
<p>Let us define the magnetic field
$$\vec{B} = g\frac{\vec{r}}{r^3}$$
for some constant $g$. How can we show that the divergence of this field correspond to the charge distribution of a single magnetic pole (monopole)?</p>
<p><strong>EDIT:</strong></p>
<p>If I calculate the divergence I get
$$
\begin{align}
\nabla\c... | g18385 | [
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<p>Imagine the following situation: I have a thin stationary water film, like a soap bubble, suspended inside a large ring. I throw a small loop of string onto the film and punch a hole inside it. How can I describe the motion of the hole in the water film bounded by the loop of string? The surface tension of the surro... | g18386 | [
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-0.... |
<p>May be it is small question in this forum but I'm trying to get the feel of the understanding about the angular velocity. If this question is getting rejected please kindly refer me to appropriate forum.</p>
<p>From the definition of angular velocity the directory vector of angular veloticy is perpendicular to the ... | g18387 | [
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<p>In the Wikipedia article on the twin pararadox, there is an interesting <a href="http://en.wikipedia.org/wiki/Twin_paradox#Difference_in_elapsed_time_as_a_result_of_differences_in_twins.27_spacetime_paths" rel="nofollow">chapter</a> which calculates the difference of age for the twins, with steps of accelerated move... | g18388 | [
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0.044861529022455215,
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<p>The title really says it all: Why is this case? A "Feynman type" answer would be really appreciated as I'm more of a layman that a physicist. </p> | g18389 | [
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<p>An old professor of mine once said that an effective means to get people interested in Physics is to get them started early. </p>
<p>What would be an effective and meaningful (and fun) means to explain Newton's 3 laws of motion to a 6 year old?</p> | g18390 | [
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<p>What is <a href="http://en.wikipedia.org/wiki/Weak_interaction" rel="nofollow">weak interaction</a>? I need easy and short answer.</p>
<p>I can't understand the definition of weak force.</p>
<p>Why is it so difficult? Bosons, Mesons, Fermions, parity etc.</p>
<p>Why the fundamental force is too hard to understand... | g18391 | [
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<blockquote>
<p><em>Cylinder A has a constant downward speed of 1 m/s. compute the velocity of cylinder B for θ= 30 and θ= 45. The spring is in tension throughout the motion range of interest, and the pulleys are connected by the cable of fixed length.</em> </p>
</blockquote>
<p>Figure: </p>
<p><a href="https://www... | g18392 | [
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<p>Recently, I performed experiments to characterise the ultraviolet-A response in a smartphone camera (with the lens still attached). This question focuses on Figure 2 of my paper <a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2012.01216.x/full" rel="nofollow">"Characterization of a Smartphone Camera'... | g18393 | [
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<p>We are always told that there are the four fundamental forces or interactions of nature: gravitation, electromagnetism, and the weak and strong forces. We know that gravitation is attractive, that electromagnetism can be attractive or repulsive depending on the electric charge of the interacting particles, and that ... | g18394 | [
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0.04867170751094818,
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0.... |
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