question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>Does spinning an object make it heavier? </p>
<p>A real-world example: </p>
<p>I was mowing the lawn in front of my house, a lawn that tends to have some steep inclines. I realized that the lawn-mower was easier to move, and much easier to pull up inclines when it was off than when it was running. The only differe... | g12948 | [
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<p>To me it seems to come out to be kg/(m*s^2). Is this somehow equivalent to N/m^2?</p> | g12949 | [
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<p>Near a magnetar with $B>B_\rm{QED}$, the strong B field will suppress the Compton scattering cross section of photons with a specific polarization (E-mode). Some references I know deal this problem using time-dependent perturbation in quantum mechanics.</p>
<p>My question is there an intuitive way in QED to see ... | g12950 | [
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<p>Following up on a comment by BlueRaja to <a href="http://physics.stackexchange.com/a/76758/8563">this beautiful answer of Ilmari Karonen</a>, I would like to phrase this follow up question:</p>
<p>How many air molecules hit your average human's skin during their lifetime? Actually, make that <strong>how many oxygen... | g12951 | [
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<p>It is obvious that there is no change in the mass of it and its radius. But the shape of the object does change. Does it mean its moment of inertia will also change?</p> | g12952 | [
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<p>I have a question relating to uncertinity.The equation used is period of a pendulum.<br>
<br>
T=2π √(l/g)<br></p>
<p>For example consider time to complete 10 cycles by the pendulum as 19.6 (+/- 0.2)s.<br>
I want to know how to calculate the <strong>uncertinity</strong> of
<br>1.period (T)
<br>2.T^2
<br>3.log T</p>
... | g12953 | [
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<p>Suppose i have some electrons stored in a empty shell container with a negative ion layer in the inner surface so the electrons keep bouncing inside without being able to leave the inner cavity.</p>
<p>I want to compute the electrostatic pressure that the electrons exert on the shell container. So i assume the elec... | g12954 | [
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<p>As I remember long ago, in my physics classes, I always had a great trouble understanding the concept of <a href="http://en.wikipedia.org/wiki/Wave" rel="nofollow">waves</a>. Our professor used to explain, as if everything in this world is made up of waves.
However, with any logic, a normal person would always conc... | g12955 | [
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<p>I was watching an old cartoon movie where a scientist makes a gadget, which when bound on the wrist, freezes the movement of the whole world. So, that one may do 100s of things in a single second. (The beagle boys later use the gadget to rob a whole bank in 1 second). </p>
<p>The scientist explains the working that... | g12956 | [
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<p>Some amateur scientist asked me that why can't one just simply apply the entire theory of QM at atomic scale to "quantize" celestial system with a different choice of $\hslash$, which he believed can be used to understand celestial orbital configurations, for example, the "discrete" distances from various Solar Syst... | g12957 | [
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<p>Consider a homogeneous isotropic universe filled with a perfect fluid with density $\rho$ and pressure $P = \rho/w$. E.g. for $w=-1$ we get a universe equivalent to one with "vacuum energy" or with the cosmological constant.</p>
<p>However, consider now that it is really a perfect fluid with which we can manipulate... | g12958 | [
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<p>In our everyday experience termperature is due to the motion of atoms, molecules, etc. A neutron star, where protons and electrons are fused together to form neutrons, is nothing but a huge nucleus made up of neutrons. So, how does the concept of temperature arise?</p> | g12959 | [
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<p>My mind has been busy recently by this question:</p>
<p>What is physics of Collision between solid and liquid (or gas)?</p>
<p>What is Conservation law's of such Collision?</p>
<p>Why, when we drop a solid object in water, water shows an Strange behavior (infact what is Conservation law's of energy and momentum w... | g12960 | [
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<p>Let's assume a setup with a static linear molecule with three identical atoms connected by bonds and a single atom, identical to the other three, being shot at the molecule. Let's also assume that everything happens in 1D and can therefore be illustrated by this simple scheme:</p>
<pre><code>(projectile) A -> ... | g12961 | [
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<p>When I vary the action of the YM Lagrangian density $$L = -\frac{1}{4} F^a_{\mu \nu}F^{\mu \nu}_a + J_a^\mu A^a_\mu$$ with respect to the metric, I obtain:</p>
<p>$$T_{\mu \nu} = \frac{-2}{\sqrt{|g|}} \frac{\partial}{\partial g^{\mu \nu}} \left( \sqrt{|g|} L \right) = \frac{1}{4} g_{\mu \nu} F^a_{\sigma \alpha} g^{... | g12962 | [
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<p>I was watching a show on the science channel about gas giants; there is something I do not understand. I am not a scientist, so this may be obvious to some. I learned that there a three states of an given physical object; solid, liquid, and gas depending on how cold or hot the object is. An easy example is ice, wate... | g12963 | [
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<p>How about changing H atom to other kinds of atoms?</p> | g12964 | [
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<p>A cube has a side length of 20 cm. An atom in the gas moves around the cube as shown. It continually bounces off the four lateral walls of the cube. The atom has a mass of 6.6×10−27 kg. Because of the elastic collisions with the walls, the atom maintains its speed of 300 m/s as it moves around. The figure below show... | g409 | [
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<p>The problem is this:</p>
<blockquote>
<p>Consider an electron confined in a region of nuclear
dimensions (about 5 fm). Find its minimum possible
kinetic energy in MeV. Treat this problem as
one-dimensional, and use the relativistic relation between
E and p.</p>
</blockquote>
<p>From Heisenberg's uncertai... | g12965 | [
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<p>This is kind of hard to explain, because weird as it sounds, i have experienced a phenomenon that i would like to see if it exists and if i can explain it mathematically.</p>
<p>The longitudinal waves of photons( visible light spectrum) had a density and pressure.
It could be sound waves if they can produce light e... | g12966 | [
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<p>A cube has a side length of 20 cm. An atom in the gas moves around the cube as shown. It continually bounces off the four lateral walls of the cube. The atom has a mass of 6.6×10−27 kg. Because of the elastic collisions with the walls, the atom maintains its speed of 300 m/s as it moves around. The figure below show... | g409 | [
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<p>Suppose I have an wave source and light waves are radiating from it. If I have a point source, then after a time t, with a radius of ct I will have a circular wave front.By Huygens principle each point on the wave front acts like a secondary source. From this point again light can travel in all directions. So isn't ... | g12967 | [
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<p>In quantum mechanics, one makes the distinction between mixed states and pure states. A classic example of a mixed state is a beam of photons in which 50% have spin in the positive $z$-direction and 50% have spin in the positive $x$-direction. Note that this is not the same as a beam of photons, 100% of which are ... | g12968 | [
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<p>How much energy would be necessary to slow down Earth rotation such that a year was 360 days long?</p>
<p>In the same spirit: how much energy would be necessary to make the Earth rotate faster around the Sun such that a year was 360 days long?</p>
<p><em>(Just for fun: how would you do it practically?)</em></p> | g12969 | [
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<p>Is it possible to unify the strong, weak, electric and magnetic field just by Maxwellian type equations?
(Maxwell by adding a small change - unified electric and magnetic field, then Einstein's equations - use it to create special and general theory of relativity, now maybe all we need is a little more change to uni... | g12970 | [
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<p>Let's say I have a uniformly-charged wire bent into a semi-circle around the origin. How can I find the electric field (magnitude and direction)</p>
<p>I'm not even sure if I should use Coulomb's or Gauss' law either. Kind of stuck setting up a solution and starting somewhere.</p> | g12971 | [
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<p>A long space train's HAL 2000 computer goes wacko and drives the ship and its sleeping crew front first straight into a Black Hole. As it nears/crosses the event horizon, does the space train break up due to gravitational stretching from the rear first, the front first, or evenly ?</p>
<p>Contrary to depictions on ... | g12972 | [
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<p>I got asked this question for my physics course at college, i'm not the best at airspeed velocities...</p>
<blockquote>
<p>What is the airspeed velocity of an unladen swallow?</p>
</blockquote>
<p>I still can't find anything to help me, i'm quite stuck and have been googling and asking about on a few forums.</p>... | g12973 | [
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<p>New results published about photonic time travel, <a href="http://www.nature.com/ncomms/2014/140619/ncomms5145/full/ncomms5145.html" rel="nofollow">reference</a> here make quantum computers a reality in the near future? These results seem to indicate that there can be qubits that can exhibit nonlinear behaviour, and... | g12974 | [
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<p>I know that the <a href="http://en.wikipedia.org/wiki/Bohr%E2%80%93van_Leeuwen_theorem" rel="nofollow">Bohr-van Leeuwen theorem</a> shows that there could be not consistent pure classical explanation of dia- and paramagnetism. </p>
<p>Does the same theorem also rule out a consistent classical theory of ferromagneti... | g12975 | [
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<p>Let's say $g=10\ \text{m/s}^2$ and it drops for $3$ seconds. For the first second it will drop $10\ \text{m}$, the second it will drop $20\ \text{m}$ ($10 + 10$) and the third second it will drop $30\ \text{m}$. Then if we add $10+20+30$ it equals $60$ but if you use the equation it equals $45$. </p>
<p>Whats the f... | g12976 | [
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<p>Many aircrafts depend on several types of static pressure sensors (barometers) to find the altitude. I assume this sensor need to be located outside the aircraft main body, so that it has a contact with the outside atmosphere to measure its pressure.</p>
<p>Now, won't the speed of the aircraft has an effect on this... | g12977 | [
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<p>Given a beamsplitter drawn below, where $\hat{a}$ and $\hat{b}$ are input modal annihilation operators, transmissivity is $\tau\in[0,1]$, and output modal annihilation operators are $\hat{c}=\sqrt{\tau}\hat{a}+\sqrt{1-\tau}\hat{b}$ and $\hat{d}=\sqrt{1-\tau}\hat{a}+\sqrt{\tau}\hat{b}$, suppose the inputs $\hat{a}$ a... | g12978 | [
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<p>First question here.
Feel free to write if I posted in the wrong place.</p>
<p>While rotating a ship as accurately as we can around some point within the ship, we can calculate the point of rotation from GPS data collected by a GPS receiver on the ship (the GPS is positioned some distance away from the point of ro... | g12979 | [
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<p>In Faraday's Induction Experiment, the e.m.f. induced in the induction coil becomes zero when the relative velocity of the coil and the magnet becomes zero. But one can also argue from a stationary observer's reference frame. Let's assume the magnet and the coil to be moving at an equal velocity.</p>
<p>For a stati... | g12980 | [
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<p>The Universe's densest objects are black holes. In the second place, there are neutron stars.<p>
So, if a neutron star compresses to its Schwarzschild radius, would it appear as a black hole? That black hole would be one of the most dense objects in the universe?</p> | g12981 | [
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<p>The aim of the Eötvös experiment was to "prove" that for every (massive) particle, the quotient $\frac{m_g}{m_i}$ is constant, where $m_g$ is the gravitational mass and $m_i$ is the inertial mass.</p>
<p><strong>The experiment:</strong></p>
<p>Consider two objects with coordinates $x(t)$, $y(t)$ and with masses $... | g12982 | [
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<p>I'm a Computer Engineering major, but when I was in high school I was a very poor student. I didn't pay attention and I didn't think I was capable of succeeding in classes like Math and Science so I just didn't care. I'm much older now and a bit wiser, and because I'm back in school, I realize I may have some pote... | g12983 | [
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<p>I was having a discussion with a friend about human tolerances of g-force. He believed that the maximum human tolerance of negative g-force is low, in the order of .5 g. I countered, saying that you're able to stand on your head and do not die. An interesting discussion ensued but there was one thing that we couldn'... | g12984 | [
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<p>For one dimensional quantum mechanics $$[\hat{x},\hat{p}]=i\hbar $$</p>
<p>Does this fix univocally the form of the $\hat{p}$ operator? My bet is no because $\hat{p}$ actually depends if we are on coordinate or momentum representation, but I don't know if that statement constitutes a proof. Moreover if we choose $\... | g12985 | [
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<p>I find some variation in the values reported for ephemeris by the various sources I have access to. For example for 2012-11-27T03:31:55 UTC I get solar declination values of <a href="http://www.wolframalpha.com/input/?i=sun%20declination%20at%203:31:55%20AM%20GMT%202012-11-27" rel="nofollow">-21.1828°</a>, <a href="... | g12986 | [
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... |
<p>I mean, in the sense that the act of closing curtains would somehow reduce the amount of heat loss of the house to the outside, thus making it warmer <em>for a given supply of heating</em>. </p> | g12987 | [
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<p>Is there any example of a symmetry breaking phase transition in a system of particles under isothermal expansion?</p> | g12988 | [
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<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/41291/why-i-think-tension-should-be-twice-the-force-in-a-tug-of-war">Why I think tension should be twice the force in a tug of war</a> </p>
</blockquote>
<p>For (A) in this image, I would think the tensi... | g410 | [
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... |
<p>We are given a graph of the position of a wave (amplitude).
How can we calculate the wavelength, frequency and the maximum speed of a particle attached to that wave?</p>
<p>We have </p>
<p><em>Speed = wave length $\times$ frequency</em>, </p>
<p><em>$W=2 \pi \times$ frequency</em> ,</p>
<p>$V_{max}=A\times W$.<... | g12989 | [
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<p>It is often seen that according to physics the light changes it's velocity according to the medium through which it is traveling. So can it be explained that why so happen?</p> | g298 | [
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<p>The title is slightly misleading. I really want to know if the randomness and probabilities observed in quantum mechanics is really just the result of a chaotic (yet deterministic) system.</p>
<p>If it is not simply a chaotic system, how do we know?</p> | g12990 | [
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<p>I'll start out with the cliche attempt in a protective shield of my dignity. I am a young highschool kid just eager to learn and understand. If I'm way off or this is already a known idea, or maybe this post is in the wrong area, I would appreciate a nonchalant and informative correction.</p>
<p>When I imagine spac... | g12991 | [
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<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/39165/linear-algebra-for-quantum-physics">Linear Algebra for Quantum Physics</a> </p>
</blockquote>
<p>Can you get all/most of the knowledge you need of Linear Algebra for QM in a one year course? I know... | g53 | [
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... |
<p>According to the paper:
A. S. Parkins and H. J. Kimble, Phys. Rev. A 61, 52104 (2000).
<a href="http://pra.aps.org/abstract/PRA/v61/i5/e052104" rel="nofollow">http://pra.aps.org/abstract/PRA/v61/i5/e052104</a></p>
<p>You can entangle position and momenta of two atoms by using entangled light. Atoms A and B are loca... | g12992 | [
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<p>brachistochrone problem: Suppose that there is a rollercoaster. There is point 1 ($0,0$) and point 2 ($x_2, y_2)$. Point 1 is at the higher place when compared to the point 2, so the rollercoaster rolls down from point 1 to point 2. Assuming no friction, we want to build a rollercoaster path from point 1 to point 2,... | g12993 | [
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<p>How do I integrate the following?</p>
<p>$$\frac{1}{\Psi}\frac{\partial \Psi}{\partial x} = Cx$$</p>
<p>where $C$ is a constant.</p>
<p>I'm supposed to get a Gaussian function out of the above by integrating but don't know how to proceed.</p> | g12994 | [
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<p>It's an old-new question (I found only one similar question with unsatisfactory (for me) answer: <a href="http://physics.stackexchange.com/questions/68381/where-did-schrodinger-solve-the-radiating-problem-of-bohrs-model">Where did Schrödinger solve the radiating problem of Bohr's model?</a>)</p>
<p>It's strange for... | g770 | [
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<p>This might be a silly question, but if every atom has its own gravitational force could atoms or molecules be attracted to each other over vast distances in the void of space if there were no other greater forces being applied? Is this similar to the cooling/slowing theory?</p> | g12995 | [
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<p>I have a question about deriving the coordinate representation of momentum operator from the commutation relation, $[x,p]= i$.</p>
<p>One derivation (ref W. Greiner's Quantum Mechanics: An Introduction, 4th edi, p442) is as following:
$$ \langle x|[x,p]|y \rangle = \langle x|xp-px|y \rangle = (x-y) \langle x|p|y \r... | g12996 | [
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<p>In section 3.4.1 of Griffiths' <em>Introduction to Electrodynamics</em>, he discusses electric multipole expansion.</p>
<p>He derives the formula or the electric potential of a dipole, which I follow, but right after, he begins talking about the electric potential at a large distance, which is as follows: </p>
<p>... | g12997 | [
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<p>I'm reading <em>Quantum Liquids</em> by A.J. Leggett and became confused by the following statement in the first chapter.</p>
<blockquote>
<p>Consider now a pair of such identical atoms. In the absence of appreciable coupling between the hyperfine and (atomic center-of-mass) orbital degrees of freedom, the energy... | g12998 | [
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0.03282153606414795,
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0.03858150914311409,
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<p>I understand that combining deuterium and tritium will form helium and a neutron. There are three methods to do this (1) tokamak (2) lasers and (3) cold fusion. I would like to know after helium is formed. How is that energy extracted from tokamak and stored? </p> | g12999 | [
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<p>Up until now I've explained relative time to myself as looking at the 3D world from different four-dimensional perspectives, analogous to how looking at a 2D-ish object (eg. a sheet of paper) from different angles makes it appear to change shape.</p>
<p>Also, I've used this representation below to explain how the t... | g13000 | [
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<p>I don't understand the following reasoning that I found in a set of lecture notes from a physics course, it's about Perrin's stimate on $N_{a}$ Avogadro's number via the bariometric formula</p>
<p>In order to stablish that formula, the reasoning begins with Boltzmann distribution</p>
<p>$$\frac{n(E)}{n(0)}=\exp\le... | g13001 | [
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<p>Topological order are sometimes defined in opposition with the order parameter originating from a symmetry breaking. The latter one being possibly described by a Landau theory, with an order parameter. </p>
<p>Then, one of the distinctions would be to say that <em>topological order can not be described by a <strong... | g13002 | [
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<p>I've seen many Young-Earth Creationists attempt to <a href="http://crev.info/2012/01/cosmologists-forced-to-in-the-beginning/">discredit</a> Big Bang cosmology. For example, an article by Cal Thomas in the 1990's stated that it meant there was a "pre-eternally existing 'cosmic egg' that decided to explode." A new re... | g13003 | [
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-0.02729680947959423,
-0.012403152883052826,
-0.04625733941793442,
0.049643006175756454,
0... |
<p>Why does there have to be a singularity in a black hole, and not just a very dense lump of matter of finite size? If there's any such thing as granularity of space, couldn't the "singularity" be just the smallest possible size?</p> | g13004 | [
0.03916914761066437,
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0.02386... |
<p>Let's say that a hot gas is trapped in a metal box. This metal box is magnetically suspended in another structure with a low temperature. The inner box does not touch anything. And there is a void in the structure. To me there seems to be no way for heat transfer to occur. Will the gas change its temperature with t... | g13005 | [
0.03432900458574295,
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0.06426898390054703,
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... |
<p>Since l=0 for a valence electron in 5s state of silver, L=0 and therefore magnetic dipole moment is also 0 which means that the beam should not have deflected at all. So, we introduced the property of an electron called spin which explains the deflection. However, my problem is How the introduction of spin explains ... | g13006 | [
0.051573917269706726,
0.0029165465384721756,
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0.0006712559843435884,
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0.03421616926789284,
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-0.... |
<p>I was looking in New Scientist the other day when I saw something to do with the Higgs boson, energy levels, entropy, space/time, quantum oscillations and many other things. It was in a feature to do with symmetry. I have read up about this and I know that the magazine does not accept original works, so I guess that... | g13007 | [
0.02342698536813259,
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<p>So in my engineering statics course we have been doing a great deal of work with trusses and machines, and a part of the physics behind it has really been confusing me. Just as a note, I also have been trying to use matrices/inversion once I have my family of equations since I also like trying to use linear algebra ... | g13008 | [
0.016732754185795784,
0.00040576455648988485,
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... |
<p>Let suppose that $\vec{F}$ is the velocity vector field of of a fluid in space, and $\vec{s}$ is a straight path of arbitrary length. Let's suppose that $\vec{F}$ and $\vec{s}$ are parallels and point in the same direction, so that $\vec{F} \cdot \vec{s} = F s$. How can we account for the total amount of fluid that ... | g13009 | [
0.07064249366521835,
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<pre><code>Firstly, I am sorry if this seems like a joke. It is not.
</code></pre>
<p>I have always loved the Dragon Ball series, so this question has been on my mind for a while.
Can any of you figure out why time would move slower in a " 'hyperbolic' time chamber?" (this seems like a relevant physics question ... | g13010 | [
0.04109753295779228,
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0.00183... |
<p>This isn't a question of how a wing works -- vortex flow, Bernoulli's principle, all of that jazz. Instead, it's a question of why we need a wing at all. A wing produces lift, but why is that necessary?</p>
<p>I got to this by thinking of an airplane at a coarse level. The wing produces lift through <a href="http:/... | g13011 | [
0.022235877811908722,
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<p>I'm hearing a guy ( Tom Cassidy ), which supposedly has a master in physics, saying that what we expect in a physical experiment ( for example, observing some particle ) can actually interefere with the workings of the experiment. Is he distorting quantum mechanics terms and facts to use it with his purpose ( what w... | g13012 | [
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<p>What does it take to become a top physicist?</p>
<p>Why do so many extremely talented young upstarts totally flop as they move to more advanced physics?</p> | g13013 | [
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<p>So I was given the following homework problem:</p>
<blockquote>
<p>You land on an unknown planet somewhere in the universe that clearly has weaker gravity than Earth. To measure g on this planet you do the following experiment: A ball is thrown upward from the ground. It passes a windowsill 15.0 m above ground an... | g13014 | [
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0.0... |
<p>At this website:</p>
<p><a href="http://heasarc.gsfc.nasa.gov/docs/swift/analysis/threads/uvot_thread_afterglows.html" rel="nofollow">http://heasarc.gsfc.nasa.gov/docs/swift/analysis/threads/uvot_thread_afterglows.html</a></p>
<p>The passage at the bottom states that a V-band magnitude of 17.62, with an error $\pm... | g13015 | [
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<p>Are strange metals described by a quantum critical theory?</p> | g13016 | [
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<p>In General Relativity, a totally rigid body cannot be accelerated. It will behave like something of infinite mass. </p>
<p>Similarly a body of two separated particles which connected to each other with a highly rigid bond, will be difficult to accelerate. The more rigid bond is, the more difficult will it be to acc... | g13017 | [
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<p>I need to verify that the solution for vanishing <a href="http://en.wikipedia.org/wiki/Weyl_tensor" rel="nofollow">Weyl tensor</a> is conformally flat metric $g_{\mu\nu} = e^{2\varphi}\eta_{\mu\nu}$. The most convenient way to show this is to prove that Weyl tensor is invariant under conformal transformation of the ... | g13018 | [
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<p>The energy level of an electron could be shifted by an electric field. $\langle n, l,m|[L_z,z]|n^{\prime},l^{\prime},m^{\prime}\rangle=(m-m^{\prime})\hbar \langle n, l,m|z|n^{\prime},l^{\prime},m^{\prime}\rangle$. Because $[L_z,z]=0$, we must have $m= m^{\prime}$ for non-vanishing $\langle n, l,m|z|n^{\prime},l^{\pr... | g13019 | [
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0.0... |
<p>If we cannot define a proper time (or synchronize clocks in different positions) in an inertial frame (independent with the theory of relativity), there seems to be no direct way to confirm the 2 postulates of Einstein. Is that true?</p>
<p>Consider an inertial frame, which is associated with a proper coordinate $(... | g13020 | [
0.025083839893341064,
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0.0... |
<p><img src="http://i.stack.imgur.com/oCVEr.jpg" alt="block-spring system"></p>
<p>I am reviewing for an exam next week, and this is one of the questions I am stuck on. I have the mass-spring system above with spring constant $k$ on a frictionless incline. I would like to find the total energy of the system at any tim... | g13021 | [
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<p>I am doing msc physics. And we are studying major part of scattering theory. I used <em>Quantum Mechanics</em> by Davydov, Griffiths, etc, to study scattering theory. But I am not understanding it properly, hence I want a specific book which will explain scattering theory very clearly. </p> | g98 | [
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<p>Having a layman level of logic about probability ( read about it 10 years ago, so pardon if it's still not right completely) what i know that the calculation requires some knowledge about the number of total possibilities vs outcomes. Say for a coin there are total 2 and 1 outcome each time. So "50%" .
So, if the ... | g411 | [
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<p>In Landau Mechanics (third edition page 4), why does adding Lagrangians of two non interacting parts remove the indefiniteness of multiplying each Lagrangian by a different constant?</p>
<p>If both systems are completely non-interacting, I can perfectly create a new Lagrangian L = a*L1 + b*L2 and still maintain val... | g13022 | [
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<p>MY thoughts: I think it will not be as the expression for RMS value is dependent only on the peak value of current. Is my thinking correct? </p> | g13023 | [
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<p>We know that when we strike a metal, it usually has a characteristic "sharp" sound, unlike when we strike wood, say.</p>
<p>What characterizes this "metallic sound"? Does it have a well-defined power spectrum? What are its generic properties?</p>
<p>Also, why do metals have a metallic sound?</p>
<p>My best guess ... | g13024 | [
0.05350759997963905,
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<p>Very simple homework question which I managed to get wrong:</p>
<blockquote>
<p>"The weight of a box sitting on the floor points directly down. The normal force of the floor on the box points directly up. Need these two forces have the same magnitude? (y/N)"</p>
</blockquote>
<p>My answer was yes, the correct an... | g13025 | [
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<p>If I only know $x$- and $y$- coordinates of every point on a trajectory without knowledge of time information, is there any way to approximate Cartesian acceleration angle at each point? Time interval between every two points is very small, ~0.03 second. </p> | g13026 | [
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<p>Step #1 Imagine one preforms an electron based Double-slit-experiment and one does so with only one electron being fired at a time. </p>
<p>Step#2 Also included in the experiment is an unobserved alternating opening of the double slits such that there is only one slit open at a time, and that each individual altern... | g13027 | [
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<p>Why are <a href="http://en.wikipedia.org/wiki/MHV_amplitudes">MHV amplitudes</a> so important? How/where are they used and why do people keep trying to rederive them in many different ways?</p> | g13028 | [
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<p>I'm just learning about relativity, and every equation I see for a galilean transformation of frame $S'$ (moving with uniform velocity in the $x$-direction with respect to frame $S$) is $x'=x-vt$, $y'=y$, $z'=z$, $t'=t$. My question involves the $x'$ term. Don't these equations assume that frames $S$ and $S'$ align ... | g13029 | [
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<p>Let's say you are observing the movement of the Sun and Earth from very far away. After year 1, the Earth creates an orbital plane around the Sun. Since the objects in the universe induce forces of different magnitudes on the Earth and Sun, you will observe the Sun and Earth moving differently, although the Earth wi... | g13030 | [
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<p>Unitary Groups is the most mysterious thing for me when studying physics. All my physics endeavor ends when author starts talking about unitary groups. This is often the case because in a lot of the texts or papers the author would throw terms like $SU(3)$, $SU(5)$ without any background motivation. </p>
<p>When yo... | g13031 | [
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<p>We created a table in my physics class which contained the strength of gravity on different planet and objects in space. At altitude 0(earth), the gravitational strength is 100%. On the moon at altitude 240,000 miles, it's .028%. And on the International Space Station at 4,250 miles, the gravitational strength compa... | g13032 | [
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<p>Is there a term referring to space that is inside the plane of a galaxy, but not part of the center/bar/arms/spurs, etc? What's the filler called?</p>
<p>The space between two spiral arms (if it isn't a spur or anything) would be called... I really feel like there should be a word for this, but I can't find one.</... | g13033 | [
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0.004934978671371937,
-0.042954470962285995,
-0.015783874318003654,
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-0.051369715481996536,
0.04240501672029495,
0.009032617323100567,
-0.027045506983995438,... |
<p>The question is: </p>
<blockquote>
<p>A 90kg disc is floating in a frictionless vacuum. A 150N force is
applied to the outer rim of the disc. The disc has a radius of 0.25m
and a radius of gyration of 0.16m. What is the acceleration and
angular acceleration of this disc?</p>
</blockquote>
<p>To solve it I ... | g13034 | [
0.046325426548719406,
0.07026731222867966,
0.009063838049769402,
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0.07492318749427795,
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0.04942190274596214,
0.0061021773144602776,
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-0.02199195884168148,
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0.02393593266606331,
-0.018418623134493828,
0.0... |
<p>So I am building a circular accelerator (like a slightly oversimplified model version of CERN) as a physics project, and I'm at a dead end. I believe I need to know how to work out differential equations to answer this, but I'd like to know if there's any other way to get the answer.</p>
<p>Okay. I am calculating t... | g13035 | [
0.024843204766511917,
0.019824007526040077,
0.012951664626598358,
-0.06739456206560135,
0.009359486401081085,
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0.05646756291389465,
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-0.0031447552610188723,
-0.05135985091328621,
0.00495834369212389,
-0.007292162626981735,
0.... |
<p>I heard capacitors affect the valleys and mounds of voltage sine curves, so that you get DC from AC. It's related to Graetz bridge, flipping signs of sine waves and seemingly afterwards smoothing sharp points, see pictures below.</p>
<p>How can that be derived and interpreted from electronics? What is the math behi... | g13036 | [
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0.0029591317288577557,
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0.009366828948259354,
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-0.019442541524767876,
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0.11539295315742493,
0.07103648781776428,
0.014900... |
<p>Two twin sisters synchronize their watches and simultaneously (from the earth frame) depart earth in different directions. Following a predetermined flight plan, each sister accelerates identically to 99.9%c and then returns home at the same time (again in the earth frame). The observers on earth see each twin as ha... | g84 | [
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0.01523278933018446,
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0.0165299940854311,
0.06325606256723404,
0.09435676038265228,
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-0.014432490803301334,
-0.01670895889401436,
0.0016597412759438157,
0.04161636158823967,
0.006... |
<p>It is said in textbooks that if the $SU(2)_f$ or $SU(3)_f$ flavor symmetry were exact for sstrong isospin, then all members of the multiplets would be exactly equally massive. By looking at quark masses then one can conclude that the symmetry is not exact. But how does the argument work with the weak force ? It is s... | g13037 | [
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0.009161551482975483,
... |
<p>How would you proof that
$$ \mathrm {Tr} (\mathbf{S\cdot \bar S })=0$$
where $\mathbf S$ is an element of area delimited for the 4-vectors $\mathbf u$ and $\mathbf v$ given by
$$S^{\alpha \beta}\equiv u^\alpha v^\beta-u^\beta v^\alpha$$
and
$$\bar S^{\alpha \beta}\equiv \frac{1}{2}\epsilon^{\alpha \beta \gamma \del... | g13038 | [
-0.004792252089828253,
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0.03356276452541351,
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0.026287619024515152,
0.025173773989081383,
-0.... |
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