# All Questions

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### Movement of man and ladder and their center of mass

Suppose there is a massless frictionless pulley. A rope over it carries a mass $M$ and on other side carries a ladder of mass $(M-m)$ and a man on that ladder, of mass $m$. Now the man starts climbing ...
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### What would be the effects if Jupiter was to blown up?

So, in one science fiction story, that tries to be realistic as possible apart from few space magics, humanity has contingency plan to blow up Jupiter. As in, totally destroy it in one massive nuclear ...
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### Diifference between real time propagation and imaginary time propagation?

Suppose I want to solve Nonlinear Schrodinger equation using imaginary time propagation to get the ground state solution. I choose $t = - i t$, and then solve the equation using split step Crank ...
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### Intuition behind classical virial theorem

I am continuing to brush up my statistical physics. I just want to gain a better understanding. I have gone through the derivation of the classical virial theorem once more. I have thought about it, ...
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### Work done by friction

Suppose we have a block of mass $M$ and we are moving it up a curve, very slowly ($a=0$). The surface is not smooth, and coefficient of friction is $\mu=\mu_s=\mu_k$. To move the block we apply a ...
Fast everyone knows the relation $E = mc^2$ but I still ask me, which units the relation has. $c$ is the velocity of light. It means, a constant, isn't it? We therefore just have the relation $E = \... 5answers 7k views ### Eye sensitivity & Danger signal [duplicate] Why are danger signal in red, when the eye is most sensitive to yellow-green? You can check luminosity function for more details... 1answer 1k views ### Net work output of an engine performing Carnot cycle? My problem gives me a Carnot cycle heat engine with water as its working fluid, with$T_H$,$T_L$, and the fact that it starts from saturated liquid to saturated vapor in the heating process. I need ... 1answer 248 views ### Can you enter a timelike hypersurface? As I understand it, a timelike hypersurface is one that has only spacelike normal vectors. But does this not imply that a the geodesic of a particle crossing it must be spacelike at that point? But ... 1answer 91 views ### In$PdV$, what is the value of$P$?$P_1$or$P_2$? Say I have an ideal gas that has a known$P_1$,$P_2$,$T_1$, and$T_2$undergoing a reversible adiabatic process. I want to find the work done so I must use$PV = RT$to get the change in$V$, so ... 1answer 1k views ### Change of coordinates from an arbitrary frame to a locally inertial frame in General Relativity If I have the following metric: $$ds^2=(1-2\phi)c^2 dt^2 - (1-2 \phi)(dx^2+dy^2+dz^2)$$$\phi$being the gravitational potential with$|\phi| << 1$everywhere. How do I find a coordinate ... 1answer 544 views ### Can a hovering helicopter travel half the globe in 12 hours? [duplicate] Suppose we have a helicopter that is able to stay stationary in flight for extended periods of time. If such a helicopter stayed at point A in the sky for 12 hours straight, would it reach the other ... 1answer 473 views ### Metronome synchronisation applied to swings The movement of several metronomes can be synchronised when a movable floor is utilised which couples the movement of the different metronomes. Is it possible to apply this sort of synchronisation to ... 2answers 3k views ### Rolling ball which slips A bowling ball of mass$M$and radius$r_0$is thrown along a level surface so that initially ($t = 0$) it slides with a linear speed$v_0$but does not rotate. As it slides, it begins to spin, and ... 1answer 489 views ### Intuition behind Work I have a doubt in understanding the intuition behind the concept of work. First of all, I think this isn't duplicate, I've searched on the site, and the closest thing I've found was this post which is ... 1answer 355 views ### What are the conserved charges related to the Virasoro generators? I have just learned from reconsidering my demystified book, that when conformally maping the worldsheet of a closed string to the complex plain by using the transformation$z = e^{\tau + i\sigma}$... 2answers 971 views ### Difficulties with understanding total entropy change and unavailabillty Of course, I know the fact that the entropy of an isolated system never decreases. Neverthless what makes me confused about the entropy(or change of entropy) of an isolated system is the explanation ... 1answer 3k views ### Archimedes principle and specific gravity A physical balance measures the gravitational mass of a body. I conducted an experiment to find out the specific gravity of a bob. I first measured the mass of the bob in air, and then in water. The ... 4answers 3k views ### Does the sun rotate? As implied from the question, does the sun rotate? If so, do other stars not including the sun also rotate? Would there be any consequences if the sun and other stars didn't rotate? Me and my friends ... 1answer 6k views ### How to understand the work-energy theorem? How to understand the work-energy theorem? I took a short lecture on physics for engineering last week. The lecturer emphasized that the work done on an object will cause the kinetic energy change as ... 1answer 734 views ### Diving into a charged (Reissner-Nordstrom) Black hole Apparently there are two event horizons in this type of black hole, where the second one is known as the Cauchy horizon. According to Carroll, if you go into the first one, you will fall until you ... 0answers 159 views ### Transfer functions of active filters I've been looking for some straightforward method or trick to obtain the transfer functions of active filters (like the Sallen-Key filter, the butterworth or Cauer topology etc...) since KCL or KVL ... 1answer 317 views ### Strings and QFT: particles moving backward in time? New question: In string theory and QFT, do particles travel back in time? Not related to antimatter: Do they travel back and forth in time in reality or are these just interpretations of mathematical ... 1answer 127 views ### Interacting particles We are familiar with the grand partition function for the grand canonical ensemble. This makes me wonder: what kinds of modifications would be required if the particles interacted? Thanks. 1answer 519 views ### Why work to change velocity from 0 to 20 km/h is less then from 20 to 40? [duplicate] Imagine spaceship in vacuum with mass = 1. At beginning, it has velocity 0, and kinetic energy 0. $$W_1 = 0$$ Then, it turns on its engine, and changes velocity from 0 to 20 (delta v = 20). It's ... 1answer 1k views ### Newton's Third Law of Motion I want to start off by saying that I've looked around for other explanations, but I've not really found any satisfying ones. My question is basically the whole "Why can anything move at all?" question,... 1answer 516 views ### Information bearing degrees of freedom of a quantum simple harmonic oscillator I am trying to make sense of arXiv:physics/0210005. I am confused with the concept of information bearing degrees of freedom of a system mentioned at the very beginning. To verify the arguments of the ... 1answer 1k views ### Why is the equation for Entropy of an ideal gas that undergoes reversible change in T at constant Pressure like this? Why is the equation for change in Entropy for a reversible change in$T$at constant$P$described as $$\Delta S = n C_p \ln\frac{T_f}{T_i}$$ 1answer 2k views ### Frictional force on a rope wrapped around a drum So there's a rope wrapped around a circular drum, subtending an angle$\theta$. A large force,$T_A$, pulls in in one direction, and a much smaller force$T_B$pulls in in another direction. The ... 1answer 661 views ### Conservation of Angular momentum in the dipole selection rules If the total angular momentum J of an atom is not changing during a dipole transition, where does the angular momentum for the photon come from? 1answer 126 views ### Pendulum axes confusion Suppose I have a simple pendulum and I want to calculate its acceleration when the bob reaches the maximal angle. I usually choose my axes such that the y-axis will be parallel to the rope. Then the ... 1answer 525 views ### Hamiltonians, density of state, BECs When working with Bose-Einstein condensates trapped in potentials, how can one tell what the density of state of a system of identical bosons given the Hamiltonian,$H$? (I have been told that it is ... 1answer 144 views ### microcanonical distribution We know that in an isolated system, the density matrix is the microcanonical distribution matrix. That this the possibility for all the states with energy in a certain interval is a constant? But how ... 0answers 107 views ### Do particles travel backward and forward in time? [duplicate] All these classical ideas are pointless and obsolete today, because in quantum mechanics, the particles are completely different objects, defined by quantum motion of fields, not by the location of ... 1answer 327 views ### Multiple vacua vs. vev's in qft Take a (possibly supersymmetric) relativistic quantum field theory: when we construct it, we suppose that there is a unique vacuum state$|0\rangle$which is Lorentz invariant, vector of some Hilbert ... 2answers 257 views ### metric tensor of expanding universe Why is the metric tensor of a expanding universe a function of time? Why is it not a function of distance between the galaxies? I heard this from a lecture. Can anyone help me understand? 1answer 336 views ### Motion of mercury [duplicate] I studied that mercury motion around the sun slightly displace by a certain value in each year. But, this is not predicted by kepler until general theory of relativity. What does general theory does ... 1answer 892 views ### Taylor expansion of an integral in spherical co-ordinates I've some difficulty deriving this equation from Jackson electrodynamics (The equation after 1.30) $$\nabla^2 \Phi_a\left({\textbf{x}}\right)=-\frac{1}{\epsilon_0}\int_{0}^{R} \frac{3a^2}{\left(r^2+a^... 1answer 241 views ### Is a holographic recorder able to capture a large full color picture? [closed] Is it practical to attempt to build a 3D hologram generator that is full color and big enough to recreate a watermelon full size? If so, is real-time control feasible? 2answers 346 views ### Inner Product Spaces I am trying to reconcile the definition of Inner Product Spaces that I encountered in Mathematics with the one I recently came across in Physics. In particular, if (,) denotes an inner product in ... 0answers 74 views ### Does the ensemble of effective Lagrangians in the String theory landscape mostly include gauge theories? String theory false vacua can be described by effective Lagrangians at low energy. Is there generally a correspondence between these effective Lagrangians and SU(N) gauge theories? Or do the effective ... 2answers 2k views ### Help me get out of a speeding ticket I was driving uphill from a complete stop for a distance of .4 miles estimated to take 1 minute in a navigation app. I was pulled over right after cresting the hill. The cop had me on radar going ... 3answers 4k views ### Why does a superconductor obey particle-hole symmetry? We normally solve the Bogoliubov-de Gennes (BdG) equations in order to compute the energy spectrum of a superconductor. The Nambu spinor is a common object that is used in formulating these equations. ... 1answer 202 views ### Vector Addition — Direction Say we have three forces F_1, F_2, F_3, such that$$ F_1 + F_2 - F_3 = 0\hspace 10mm (1)$$And let us say that$F_1$and$F_2$have the same direction and magnitude, and that$F_3$has double the ... 1answer 70 views ### Conversion of a star I need to know what would happen to a star that has size 2 times the solar mass. I guess it would either be a neutron or red giant. Is that right? 1answer 164 views ### What exactly does this news story mean by “magnetic charge”? I recently saw this article in my Google News page, whose headline and first paragraph are: Antimatter's Magnetic Charge Revealed Scientists say they've made the most precise measurements to ... 1answer 234 views ### Why should multiple versions of a weak measurement preclude it from being a measurement of intrinsic properties of some system I've been trying to understand Stephen Parrott's criticisms of weak measurement, outlined most concisely here: http://arxiv.org/abs/0909.0295 One of his major criticisms is that weak measurement is ... 1answer 128 views ### Landauer's principle vs Wien's displacement law Can we argue based on Landauer's principle that if one bit information is changed inside a blackbody, the total radiated energy should be at least or in order of kTln2? If it is so, can we also argue ... 1answer 121 views ### About Vanishing of BRST commutator in path integral In Witten's paper Topological Quantum Field Theory, about formula (3.2), the property$<\{Q,\mathcal{O}\}>=0$depends on the assertion that$Z_{\varepsilon}(\mathcal{O})= \int \mathcal{D}X \exp(\...
Can we argue based on Landauer's principle that if one bit information is changed inside a blackbody, the total radiated energy should be at least or in order of $kTln2$? If it is so, can we also ...