question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>If two identical masses are somehow "released" into deep space (that is, they're subject to no other gravitation forces but their own, and are initially at rest to each other). What decides whether they collide or orbit each other? I'm imagining for example two 1 kg masses initially at rest, say 10 m apart... | 1,495 |
<p>we vistited yesterday a fun park and saw a ride which was a big centrifuge only in vertical.
this one had a speed meter but it was broken and i also wanted to know how much g-force is on there...</p>
<p>so my first thought was to calculate this with the centrifugal-accerleration. I have $\omega = \frac {0.5*2\pi} 1... | 1,496 |
<p>Not having studied General Relativity, I have sometimes been puzzled by references to the behaviour for "classic" black holes — as they are popularly portrayed — as being true for black holes which are not rotating and which have no charge. I don't understand the role rotation <em>does</em> play, but at ... | 1,497 |
<p>I am reading up on the Schrödinger equation and I quote:</p>
<blockquote>
<p><em>Because the potential is symmetric under $x\to-x$, we expect that there will be solutions of definite parity.</em></p>
</blockquote>
<p>Could someone kindly explain why this is true? And perhaps also what it means physically?</p> | 1,498 |
<p>when firing a proton (for example) to an atomic nucleus, from a distance $D$, the deflection angle of the proton $\alpha $ to the type of changes atomic nuclei? or always constant?</p> | 1,499 |
<p>It's actually a teaching conflict at my school. They said that $$\text{Flux}=\frac{q}{\varepsilon_0}.$$ Say for a point charge at the centre of the sphere and let's say we not put water into the sphere, so now $E/ \varepsilon_0=K$. Thus $E= \varepsilon_0 K$.
So flux becomes $q/\varepsilon_0 K$. That means that any c... | 1,500 |
<p>Suppose we have a parallelepiped shaped box full of water on the surface of the sea. Suddenly the box disappears. What is the shape of the waves vs. time caused by the fall of the water contained in the box?</p> | 1,501 |
<p>One of the most famous experiments in quantum mechanics in the context of wave-particle duality is certainly passing a beam of electrons through two slits, which results in an interference pattern being formed on a screen positioned after them.</p>
<p>Now, <strong>starting from the Schrödinger equation</strong> (wh... | 1,502 |
<p>This is a historical question partly, and maybe too broad for this site, but would require some familiarity with modern physics research practice so hopefully appropriate here. </p>
<p>Einstein's later years, after general relativity, were (as I understand it) taken up predominantly by his effort to find a unified ... | 1,503 |
<p>In QFT the principle of stationary action states that we choose fields that will make the action stationary but what if the action has many stationary points? What's the significance of these other solutions?</p> | 1,504 |
<p>Through a bit calculation we can derive that in a cavity, the energy density $$u(f,T)=\overline{E(f)}\times G(f)=\frac{8\pi h}{c^3}\frac{f^3}{e^{h\nu /kT}-1}$$
If we take the integral over all frequency, we get
$$U(T)=\frac{8πh}{c^3}\frac{(kT)^4}{h^3}{\frac{π^4}{15}}=C_{onst}T^4$$
And Stefan-Boltzmann Law claims tha... | 1,505 |
<p>I have known the reason why skate can slide over ice is that water's melting curve in terms of pressure and temperature has a negative slope. If the pressure due to our mass increases sufficiently high, the ice starts to melt.</p>
<p>But someone says, according to latest research, it is not true because the time fo... | 22 |
<p>The articles I found on radiation in the solar system mostly dealt with solar wind, I wonder about other types. Is there a breakdown that tells me, withhin an order of magnitude, at least what intensity I can expect for a the different spectra - hard gamma through to radio frequencies, and chargede particles - and h... | 1,506 |
<p>I've been reading up on nuclear reactors, and understand explanations of how it works, how water is heated to steam, which turns the turbines, etc.. I understand all of the safety features, and how control rods are used, and what they do with spent fuel.</p>
<p>However, what I can't figure out (after lookin at many... | 1,507 |
<p>We know that a piece of ferromagnet, such as iron, can be magnetized by putting in a strong magnetic field to get domains parallel to the field grow. </p>
<p>I also remember from pop. culture and MacGyver old tv series that you can magnetize a piece of iron by hitting it hard, with a hammer say, along the same dire... | 1,508 |
<p>We all know the effects of speed on time, and we have a formula for that, but I have a hunch, that time also is relative to pressure, and that by increasing the pressure on matter, in fact time will speed up for that matter. </p>
<p>I am not a physics expert ( I have a Masters Degree in Computer Engineering, and ha... | 1,509 |
<p>I read about Goodwin's "proof" that $\pi = 3.20$, it's BS and I know that. What I am wondering is whether his technique may have stumbled on something ( a warped space) years before Einstein. So I guess my question is this: </p>
<p>Is it possible that somewhere in the universe (say the vicinity of a neutron star) w... | 1,510 |
<p>How can I recognize clearly when I deal with capacitors in parallel or capacitors in series? Can follow a "rule" or a intuitive method?
I have to do difficult exercises regarding system of capacitors so I must understand when I have capacitors in parallel or in series. Can you explain me in a simple way?</p> | 1,511 |
<p>I have been asked this question by a student, but I was able and in the same time incapable to give a good answer for this without equations, so do you have ideas how one can explain this in a simple way? </p>
<p>(Answers like we can take time as an imaginary, or our space is actually pseudo-Euclidean will be hard ... | 1,512 |
<p>I'm thinking about trying to do a numerical simulation of some very simple QM problems.</p>
<p>How much space do I need? To simulate the Hilbert space?</p>
<p>I'd like to eventually simulate the absorption or emission of a photon by a hydrogen atom. So at least three particles (two fermions, one boson). Let's ... | 1,513 |
<p>I wanted to know why there is a hole in the ball (basketball, volleyball, handball) to fill the ball with air. Why can't the ball come with filled in air and fully sealed so that there is no loss of air?</p> | 1,514 |
<p>I'm wondering if there is a textbook that describes the handiwork of a particle physics analysis. There are a bunch of books about theory, about the experimental aspects like detectors, and about statistical methods, but I haven't seen one that focused on the actual work of an analyzer.</p>
<p>I mean stuff like:</p... | 1,515 |
<p>I was studying a <a href="http://en.wikipedia.org/wiki/GRE_Physics_Test" rel="nofollow">GRE Physics Test</a> problem where optical light with a wavelength of 500 nm travels through a gas with refractive index $n$.</p>
<p>If we look at the equations for wave motion and index of refraction</p>
<p>$$c=\lambda_0\... | 1,516 |
<p>One can expand any periodic function in sines and cosines. When calculating the coefficients $a_0$, $a_n$, and $b_n$ one find that $a_1>a_2>...>a_n>...$, similarly for $b_n$. </p>
<p>Is there an intuitive reason to understand this, I mean why would one expect this to happen? It looks miraculous and myst... | 1,517 |
<p>Here is the problem:</p>
<p><img src="http://i.stack.imgur.com/iojG5.jpg" alt="Here is the problem"></p>
<p>In the above figure I want help on finding the potential difference between X and Y.
It is getting quite confusing due to the battery in the middle. I found the current in both the loops using Kirchhoff's Vo... | 1,518 |
<p>I have often seen diagrams, like <a href="http://en.wikipedia.org/wiki/Lens_%28optics%29#Imaging_properties">this one</a> on Wikipedia for a thin convex lens that show three lines from a point on the object converging at the image. Do all the other lines from that point on the object that pass through the lens conve... | 1,519 |
<p>I was at a lecture yesterday and there was a demo of a van de graff generator. He held the smaller metal globe which is attached by a wire, about 4cm from the main globe. This created a spark between the two globes.
At the same time the spark happened the speakers in the hall would click loudly, the projector (on t... | 1,520 |
<p>As far as I've read online, there isn't a good explanation for the <a href="http://en.wikipedia.org/wiki/Born_rule" rel="nofollow">Born Rule</a>. Is this the case? Why does taking the square of the wave function give you the Probability? Naturally it removes negatives and imaginary numbers, but why is it the square,... | 23 |
<p>While reading a paper I ran into this particular way of writing a $\cal{N}=3$ fields (in a theory with $N_f$ hypermultiplets) that I couldn't relate to anything I had seen before in the text-books (typically Weinberg's) The claim is that $q^{Aa}$ and $\psi_{\alpha}^{Aa}$ are scalar and fermions such that $a,b$ are $... | 1,521 |
<p>A thin spherical shell made of plastic carries a uniformly distributed negative charge of -Q coulombs. Two large thin disks made of glass carry uniformly distributed positive and negative charges S coulombs and -S coulombs. The radius of the plastic spherical shell is R1, and the radius of the glass disks is R2. The... | 1,522 |
<p>Can one measure the Higgs field? Or is the higgs field not measurable?</p>
<p>I know that the higgs boson is a indication for the presence of the higgs field. But it is the only way of the presence of the higgs field?</p>
<p>When the higgs field is not measurable, what is the reason?</p> | 1,523 |
<p>Really short question, but I cannot find anything on the internet.</p>
<p>What is meant <em>proton polarisation</em>?</p>
<p>Is it to do with the spin of the proton?
I guess the spin of the proton is obtained from the vector addition of the 3 quarks' individual spins, so it can't be 0...
Can we have <em>unpolarise... | 1,524 |
<p>Approaching the speed of sound in an aircraft is relatively difficult, because the closer you get to Mach 1, the denser the pressure is around you (sound accumulates causing vibrations).</p>
<p>Is there a similar effect as you approach the speed of light? Does the Doppler Effect apply for light as it does with soun... | 1,525 |
<p>(I apologize if this question is too theoretical for this site.)</p>
<p>This is related to the answer <a href="http://physics.stackexchange.com/a/11124">here</a>, although I came up with it independently of that. $\:$ Suppose we
<br>
have a unit mass planet at each integer point in 1-d space. $\:$ As described in t... | 1,526 |
<p>We know that the light gets refracted when it enter a medium low/high refractive index. </p>
<p>But why light is not refracted when it comes out from the vertex of a prism.</p> | 1,527 |
<p>Consider the quantum system $\mathcal{B}(\mathbb{C}^d\otimes\mathbb{C}^d)$ and $|\psi\rangle=\frac{1}{\sqrt{d}}\sum_{i=0}^{d-1}|i,i\rangle$ be the (standard) maximally entangled state. Consider the state </p>
<p>$\rho_\lambda=\lambda \frac{\mathbb{I}_{d^2}}{d^2}+(1-\lambda)|\psi\rangle\langle\psi|.$</p>
<p>Now for... | 1,528 |
<p>I'm currently learning about symmetry between particles. For a simple case of two non-interacting particles at $x_1$ and $x_2$, we know that the wavefunction can be written as $\psi_{n_1, n_2} = \phi_{n_1}(x_1) \phi_{n_2}(x_2)$</p>
<p>Let's consider two cases in ground state and first excited state.</p>
<p>(a) Two... | 1,529 |
<p>The force for revolution of earth is provided by the gravitational force of attraction between earth and the sun. What provides the force(torque) for the rotation of earth? </p> | 1,530 |
<p>What kind of significant impacts have originated from $E=mc^2$.</p>
<p>Generally, it is regarded as the most famous equation of all time. Except for nuclear energy (fission and fusion) I do not know any other way in which this equation has made an impact on the world. </p>
<p>Can somebody list some developments an... | 1,531 |
<p>The process of exchange of energy between a photon and an electron only occur after a specific energy called work-function of the material. Thus, the energy transferred is quantised due to the fact that the electron can reside only in quantised orbits. </p>
<p>But, why doesn't the process happen in steps? I mean th... | 1,532 |
<p>Why is electron presented in books, pictures as a sphere, when in fact it's not?</p> | 1,533 |
<p>Is it possible to know whether a lightning bolt travel from the ground to the sky or from the sky to the ground? </p>
<p>Alternatively, it could be both sides approaching</p> | 1,534 |
<p>I'm having trouble understanding what a problem I have is seeking.</p>
<p>To simplify the problem:</p>
<blockquote>
<p>A particle reaches a speed of 1.6 m/s in a 5.0 micrometer launch. The speed is reduced to zero in 1.0 mm by the air. Assume constant acceleration and find the acceleration in terms of g during a... | 1,535 |
<p>How does the nonlinear absorption coefficient depend on the band gap?
How can that coefficient be calculated theoretically? (Preferably with an example)</p> | 1,536 |
<p>The latent heat of vaporization of water is what we normally use to calculate the heat transfer that occurs at 373 K when liquid water transits to vapor phase. But there are curves for latent heat of vaporization at different temperatures. What is meant by latent heat of vaporization at a temperature different from... | 1,537 |
<p>If we consider a classical field theory for a massless particle of integer spin $s$, in a curved space-time, one finds that it is "naturally" conformal in a space-time of dimension $2+2s$</p>
<p>For instance, the free massless scalar field theory is conformal in $2$ dimensions, the electromagnetic field theory is... | 1,538 |
<p>I have the cross-sections as a function of $\sqrt{s}$ for a process with a $u$-quark and $u$-antiquark in the initial state (eg.: $u \bar{u} \to e^- e^+$). I have a standard parton distribution function table (say, CTEQ). With these, how do I find the corresponding cross-sections for a process with protons in the in... | 1,539 |
<p>It is said that the universe is expanding and the galaxies are moving apart. I understand that the space between every two galaxies is increasing. Doesn't this seem to imply that the galaxies will have relative motion and so have a moving velocity. According to relativity and the modern belief in physics, it is said... | 1,540 |
<p>To derive Bose-Einstein and Fermi-Dirac distribution, we need to apply grand canonical ensemble:$Z(z,V,T)=\displaystyle\sum_{N=0}^{\infty}[z^N\sideset{}{'}\sum\limits_{\{n_j\}}e^{-\beta\sum\limits_{j}n_j\epsilon_j}]$. There is a constraint $\sideset{}{'}\sum\limits_{\{n_j\}}$ for quantum particles(bosons and fermion... | 1,541 |
<p>The figure shows an LR circuit with a switch and a 240-volt battery. At the instant the switch is closed the current in the circuit and the potential difference between points a and b, Vab, are</p>
<p><img src="http://i.stack.imgur.com/Xs9ak.png" alt="enter image description here"></p>
<p>Choices : </p>
<pre><cod... | 1,542 |
<p>The spatially flat FRW metric in cartesian co-ordinates is given by:
$$ds^2 = -dt^2 + a^2(t)(dx^2 + dy^2 + dz^2)$$
As I understand it there are Killing vectors in the $x$, $y$, $z$ directions implying that momentum is conserved but there is no Killing vector in the $t$-direction which implies that energy is not cons... | 1,543 |
<p>The Lagrangian of the Yang-Mills fields is given by
$$
\mathcal{L}=-\frac{1}{4}(F^a_{\mu\nu})^2+\bar{\psi}(i\gamma^{\mu}
D_{\mu}-m)\psi-\frac{1}{2\xi}(\partial\cdot A^a)^2+
\bar{c}^a(\partial\cdot D^{ab})c^b
$$
where the metric is $(-,+,+,+)$, and the conventions are the following:
$$
[D_{\mu},D_{\nu}]=-ig... | 1,544 |
<p>Any finite & non empty set of masses has a computable center of gravity:
$\vec{OG} = \frac{\sum_i m_i \vec{OM}_i}{\sum_i m_i}$ .</p>
<p>Does the contrapositive permits to conclude that a mass system with
physical evidence that it doesn't have a gravity center is an infinite
set of mass (i.e. of cardinal larger ... | 1,545 |
<p>Why do the propellers on helicopter appear to be so slow when the propellers is going at full speed? Can you please explain this particular optical illusion to me? </p> | 1,546 |
<p>$u^{\mu}$ - 4-velocity</p>
<p>$b^{\mu}$ - 4-vector of magnetic field</p>
<p>$
u_{\mu}u^{\mu}=-1, \qquad u_{\mu}b^{\mu}=0
$</p>
<p>$$
u_{\beta}u^{\alpha}\nabla_{\alpha}b^{\beta}-u_{\beta}b^{\alpha}\nabla_{\alpha}u^{\beta}+\nabla_{\alpha}b^{\alpha}=0
$$
I don't understand why this equation gives this
$$
u^{\alpha}u... | 1,547 |
<p>OK, so my textbook says that in time dilation and length contraction, the proper time and the proper length is "That which is in the frame of reference of the observer at rest relative to the event". </p>
<p>This means that the proper time/length is that which is observed by the observer moving close to the speed o... | 1,548 |
<p>I would like to know how one could show and prove that a given motion is simple harmonic motion.</p>
<p>Once given an answer, I'll apply that technique to an example I am trying to figure out. </p>
<p>Thank you in advance!</p>
<p>I believe a motion can be proved simple harmonic, if the relation between its is as ... | 1,549 |
<p>David Albert is a philosopher of Science at Columbia. His book "Time and Chance" includes this example (p 36). </p>
<p>A gas is confined on one side of a box with a removable wall. "Draw the wall out, <em>slowly</em>, and <em>perpendicular</em> to its surface, like a piston. Now it happens to be the case, it happen... | 1,550 |
<p>I've been told to revise the derivation that proves $\frac{\mathrm{d}P}{\mathrm{d}r} =\frac{GM(r)p(r)}{r^2}$ where brackets indicate a function of, P is pressure and p is density. Rather helpfully he hasn't given us it to revise, so if anyone knows it I'd be really grateful. Thanks. </p> | 1,551 |
<p>With all the recent discoveries of supermassive black holes being at the center of nearly every galaxy, and the proposed "at least one planet" around (probably) every star.</p>
<p>Does this affect the estimated total mass of our universe?</p>
<p>Is Dark Matter necessary when planets and black holes are included in... | 1,552 |
<p>An object of mass m is slowly lowered into a black hole of mass 1000 m. Is the amount of braking energy larger than $0.6 mc^2$? </p>
<p>Now what if, after lowering the mass close to the event horizon, we wait the black hole to shrink to mass 100 m, by emitting Hawking radiation, and then we lower the mass some more... | 1,553 |
<p>Well, that's easy: the sand is wet, and my shoes are wet, and hydrogen bonding adheres the wet sand to my wet feet and to my shoes.</p>
<p>But then I walk home, and my shoes dry, and the sand on them dries, and some of the sand falls off. But some does not. It's really stuck: even several days later I can turn th... | 1,554 |
<p>Can anyone give an example of when infinite-dimensional <a href="http://en.wikipedia.org/wiki/Hilbert_space" rel="nofollow">Hilbert spaces</a> are required to describe a physical system? The standard answer to this question is yes, and I'm sure some of you will be quick to point out several clear examples:</p>
<p>E... | 1,555 |
<p>Let's consider an electron-positron pair with total spin equal to zero. When it annihilates it can not emit only one photon because it would have zero momentum and nonzero energy. The pair emits two photons with opposite momenta but on the momentum-energy plain it looks like the particle goes through a forbidden sta... | 1,556 |
<p><img src="http://i.stack.imgur.com/v7mfP.png" alt="enter image description here"></p>
<p>The question asked was "what should be the acceleration such that the pressure at both the points marked by thick dots be equal? the vessel is <strong>open</strong> and cubic with side 5m?" </p>
<p>Initially i considered the ... | 1,557 |
<p>My question is fairly simple, but I do need clarification on how to get the inverse of the <a href="http://en.wikipedia.org/wiki/Lennard-Jones_potential" rel="nofollow">Lennard-Jones</a> potential V(x).</p>
<p>I am working with the following expression:
$$
V(x) = e\times[(R/x)^{12} -2\times(R/x)^6]
$$</p>
<p>So gi... | 1,558 |
<p>I'm currently consuming a course on QFT where we need to define the unitary time-evolution to get the time evolution of the wave function in the interaction picture:</p>
<p>$\hat{U}(t_1,t_0) = \exp\left(\frac{i}{\hbar}\hat{H}_0t_1\right)\exp\left(-\frac{i}{\hbar}\hat{H}(t_1-t_0)\right)\exp\left(-\frac{i}{\hbar}\hat... | 1,559 |
<p>In the 19th century, most astronomers adopted an island universe model, in which our galaxy was the only object in an infinite space. They didn't know that the "spiral nebulae" were other galaxies. This model had the advantage of being dynamically stable (unlike Newton's infinite and uniform cosmology), and of avoid... | 1,560 |
<p>In the last 40 years (approximately) people have been "discovering", "rediscovering" and "studying" SUSY as a powerful tool and "symmetry principle". </p>
<p>Question: </p>
<p>What if SUSY is not realized in Nature at the end? Is SUSY the only path to "relate" fermions and bosons or what else? Remark: SUSY has not... | 1,561 |
<p>So electrons of specific atoms have a minimum amount of energy needed to escape the atom, called the work function, W. Now let's say that you emit a certain frequency of light, and $hf<W$. However, from my understanding, this work function is just a form of kinetic energy, and if an electron has enough kinetic en... | 1,562 |
<p>I have this problem. I have an ideal gas that goes through an irreversible adiabatic decompression. I have the initial state (P,T,V), and the final pressure, and I have to calculate the entropy difference of the proccess. So, what I know is that I can make up any reversible proces bewteen the initial and final state... | 1,563 |
<p>I am having trouble understanding electromagnetic radiation (or waves in general, be it EM or sound). If I have a 1 Watt speaker, is it infinitely divided and spread out so that everyone in every direction around the speaker can hear it?</p>
<p>I do not believe they have "height", to reach more than one person at o... | 1,564 |
<p>I often see the term "net mechanical efficiency" used in literature, but I am not quite sure what it means, and what the difference is between it and "normal" efficiency. Take this sentence for example: <code>... increased the effectiveness, while reducing net mechanical efficiency.</code> What does exactly does thi... | 1,565 |
<p>It is usually said that existence of discrete spacetime violates Lorentz symmetry. What quantity is used to quantify such violation? I mean could someone points a reference for a derivation that shows such analysis.</p>
<p>My other question is that: if, and this is a big fat if, OPERA result was true (and I believe... | 1,566 |
<p>I'm having some trouble calculating the 2nd order energy shift in a problem.
I am given the pertubation:</p>
<p>$\hat{H}'=\alpha \hat{p}$,</p>
<p>where $\alpha$ is a constant, and $\hat{p}$ is given by:</p>
<p>$p=i\sqrt{\frac{\hbar m\omega }{2}}\left( {{a}_{+}}-{{a}_{-}} \right)$,</p>
<p>where ${a}_{+}$ and ${a}... | 1,567 |
<p>In Statistical Mechanics, we often postulate that for an isolated system, the phase-space density of all accessible microstates (i.e all microstates consistent with the energy) is the same. This is equivalent to assuming that the system is ergodic. This postulate leads us to the assertion that at any given time, the... | 1,568 |
<p>I'm wondering if there is more than the empty theory (no local fields, identically vanishing stress energy tensor) that can have central charge $c$ equals to $0$?</p>
<p>My intuition tells me no, the stress energy tensor would transform as a primary field and so would only contain primary fields but there are none ... | 1,569 |
<p>Why does production of white light using a LED require combining a short wavelength LED such as blue or UV, and a yellow phosphor coating?</p>
<p>Why can't a single LED produce pure white light?</p> | 1,570 |
<p>Suppose you have a U tube filled with a liquid. The height of the liquid column in both the arms of the U tube is not the same. Will the pressure be the same at two points in the two arms of the U tube which are at the same height from the base of the U tube or will it vary depending on height of liquid column above... | 1,571 |
<p>If I try to add up neutrino masses (let's assume 1 eV rest mass equivalent each) to count as DM, do I use the rest mass or relativistic mass?</p> | 1,572 |
<p>About the meta questions <a href="http://meta.physics.stackexchange.com/questions/1193/full-migration-of-theoretical-physics-se-questions-to-physics-se">1193</a> and <a href="http://meta.physics.stackexchange.com/questions/2609/why-theoretical-physics-has-been-closed">2609</a>, I've heard parallelly, that the comple... | 1,573 |
<p><strong>The Setup</strong>: Let's say we want to study a Euclidean $\mathrm{CFT}_2$ on $\mathbb R^2$ with coordinates $\sigma^1$ and $\sigma^2$ and metric </p>
<p>$ds^2 = (d\sigma^1)^2+(d\sigma^2)^2$. </p>
<p>It seems to me that in the usual discussion (e.g. di Francesco, Ginsparg, Polchinski), one proceeds to co... | 1,574 |
<p>I'm trying to learn how to apply WKB. I asked a <a href="http://physics.stackexchange.com/questions/90157/how-to-apply-the-wkb-approximation-in-this-case">similar question</a> already, but that question was related to finding the energies. Here, I would like to understand how to find the wave functions using WKB.</p... | 1,575 |
<p>Is there any temperature dependence of relaxation time in impurity scattering of conducting electrons? It seems to me that there is none. But, some people claim that there is.</p>
<p>So if you could explain, how temperature dependence comes into play if it does at all?</p> | 827 |
<p>The problem I am working on is:</p>
<blockquote>
<p><em>Big Ben, the Parliament tower clock in London, has an hour hand 2.70 m long with a mass of 300 kg, and a minute hand 4.20 m long with a mass of 100 kg (see figure below). Calculate the total rotational kinetic energy of the two hands about the axis of rotati... | 1,576 |
<p>I understand the mathematics of commutation relations and anti-commutation relations, but what does it physically mean for an observable (self-adjoint operator) to commute with another observable (self-adjoint operator) in quantum mechanics? </p>
<p>E.g. an operator $A$ with the Hamiltonian $H$?</p> | 651 |
<p>Please can anyone explain the concepts of <a href="http://en.wikipedia.org/wiki/Electromagnetically_induced_transparency" rel="nofollow">electromagnetic induced transparency</a>? I am having problem with the technicality of the explanation on wikipedia. Please I am an engineer with a physics background though and wo... | 1,577 |
<p>Photons have no mass but they can push things, as evidenced by <a href="https://en.wikipedia.org/wiki/Laser_propulsion">laser propulsion</a>. </p>
<p>Can photons push the source which is emitting them? If yes, will a more intense flashlight accelerate me more? Does the wavelength of the light matter? Is this practi... | 483 |
<p>I'm reading that general relativity let's us describe physics from the point of view of both inertial and accelerated observers. What does that actually mean in terms of doing actual physics? For example, say a physicist performed Millikan's oil drop experiment or Young's double slit experiment or decomposed white l... | 1,578 |
<p>The anthropic principle has become a very popular explanation among theoretical physicists lately. Life is unlikely, but only life can observe, so we find ourselves in an unlikely position, so to speak. What I would like to ask is just how unlikely is life? Is the evolution of life generic for most laws of physics, ... | 1,579 |
<p>I was recently reading <a href="http://en.wikipedia.org/wiki/Siphon#Explanation_using_Bernoulli.27s_equation" rel="nofollow">the explanation for the behaviour of a siphon</a> on Wikipedia, which uses Bernoulli's equation in its proof. The argument is generally pretty easy to follow, except for one key point: how can... | 1,580 |
<p>Amorphous metals are often referred to as metallic glasses due to their quenched atomic disorder. Do they fracture in the same fashion as silicate glasses? If not, what failure mode(s) do they have?</p> | 1,581 |
<p>When comparing two light sources, for example, a light bulb at 20W and a light bulb at 100W, what is it about the incoming light that makes the latter look brighter than the former? Are there different reasons why different light sources looks different in brightness (High five for cramming three instances of "diffe... | 1,582 |
<p>Does the internal structure of a neutron star resemble anything like that of an atomic nucleus? I.e., are the neutrons arranged in a shell like structure with different energy levels, and without a distinct location for each neutron?</p>
<p>I know that the force holding a neutron star is gravity, compared to the st... | 1,583 |
<p>While watching the first 4 seconds of <a href="http://www.gizmodo.com.au/2011/11/driving-at-745kmh-is-ludicrous-from-any-angle/" rel="nofollow">driving at 745 km/h is ludicrous from any angle</a>
wondered </p>
<p>1)If we knew the curvature of the earth in a "flat" desert, what would be the speed of the car?</p>
<... | 1,584 |
<p>The book I'm reading about optics says that an anti-reflective film applied on glass* makes the glass <em>more</em> transparent, because the air→film and film→glass reflected waves (originated from a <a href="http://en.wikipedia.org/wiki/Paraxial_approximation" rel="nofollow">paraxial</a> incoming wave) interfere de... | 1,585 |
<p>For some electric devices, like a fan or air conditioner, I read about their power consumption in watts on their specification guide.</p>
<p>Does it tell about the power at normal or full speed? or Does the speed even affect the power consumption? Can I find the consumption at different speeds?</p> | 1,586 |
<p>Why is electricity not transmitted wirelessly such that we don't need to span cables on the earth's surface? As in: electricity is transmitted wirelessly from the power plant to the household.</p> | 1,587 |
<p>The book I'm reading about optics says at some point that "each color (wavelength) contained in the white light interferes only with itself". But why is this so?</p>
<p>Edit: I moved the rest of the question <a href="http://physics.stackexchange.com/q/12208/4283">elsewhere</a>.</p> | 1,588 |
<p>Could anyone suggest a recent review article or book on gas chromatography instrumentation? My interest is in the devices themselves, rather than using them for a particular application.</p>
<p><a href="http://rsi.aip.org/resource/1/rsinak/v61/i11/p3317_s1" rel="nofollow">This article</a>, "Gas Chromatography" in R... | 1,589 |
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