# All Questions

136,341 questions
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### General Relativity Lorentz-like equation

In the literature, it says that, in the weak-field, $$g_{µν} = η_{µν} +h_{µν},$$ slow-motion limit, the Geodesic equation reduces to the Lorentz-like equation. Can anyone explain this?
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### Why do we define Energy density

So we define the electrical energy density $(\bar U)$ between a parallel plate capacitor as $$(\bar U) = \frac{1}{2} \epsilon_0 E^2$$ where $E$ is the electric field between the plates usually given ...
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### confusion regarding rolling without slipping

I'm trying to solve the following problem: Where the rolling uniform disk of mass M and radius has initial velocity of $v_0$ and angular velocity $\omega_0$ (it is rolling without slipping, but the ...
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### Force due to motion around Sun [on hold]

Think of one person on earth named A . Ignoring Earth for a second, I can imagine him moving around the Sun due to which he should always feel some acceleration towards Sun (Centripetal Force) say F1. ...
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### Bending moment in a cantilever beam

If I have a cantilever beam of length L fixed at the left end to a wall and I hang a weight W from it's right free end then why should the bending moment at a point x units to right of the wall be W(L-...
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### When can quantum states be distinguished with certainty?

Through a few examples, I'd like to learn under what circumstances can different quantum states be distinguished from each other. So for example, the standard quantum teleportation scheme starts out ...
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### Is it there any physics-basis for Tegmark's hypothesis? [on hold]

Tegmark's mathematical universe hypothesis is very interesting (https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis) but it has virtually no support among physicists because it's too ...
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### What kind of wave is light? [duplicate]

When people say about wave-like nature of a photon, what do they mean: Pressure-like wavefront - it moves as a sphere (or a bulge on a surface) and then if we measure pressure at one single point (...
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### General covariance and a running cosmological constant

A running cosmological constant $\Lambda(t)$ can always be included in the perfect fluid source tensor. By the transformation properties of tensors Einstein's field equation is still independent of ...
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### Why we get relativistic momentum without length contraction?

My last question was wrongly duplicated. We've already known that "p=γmv" . v is particle's velocity depends on our rest frame and mass too. But shouldn't we consider particle's momentum by its ...
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### Charge conjugation opearation in odd dimension

In my problem, I use the following set of $\gamma$-matrices (in (2+1) spacetime): $$\gamma^0=\sigma^1;\quad \gamma^{1}=i\sigma^2;\quad \gamma^{2}=i\sigma^{3},$$ where $\sigma^{(i)}$ are usual Pauli ...
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### Ordered arrangement of clouds

I happened to take a photograph of a region of clouds ,and it seems to have a rather ordered arrangement ,if one looks towards the left of the image. Would there be a physical explanation for this or ...
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### Thermal expansion of a hole in a plate with a temperature gradient

I have a rectangular metal plate with a hole in it (with diameter of 300 mm). The plate has a temperature gradient going from one of its short sides to the other, I can measure the temperature ...
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### Stale file handle error [migrated]

I am doing particle physics. I am running code on monte carlo samples. I am getting the "Stale file handle" error only when I run on one specific sample. The rest of them is OK. A screen shot of ...
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### Do we consider an object with charge to have negligible mass when we define the concept of electric potential?

In the photo I have made a fixed charge +Q . In the book I read the electric potential at infinity is 0 and to bring charge from infinity to point we have to do work against the repulsive force . And ...
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### A fictitious force in an orbiting frame of reference that is not rotating

Say we have an inertial frame of reference where there's a motionless star at the center and a planet is in circular orbit around it. We further assume that the planet is not rotating in the inertial ...
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### How much current can be produced by Van de Graaff generators or Pelletrons?

The Van de Graaff generator is a source of high voltage at low current based on the transfer of static electricity, and its higher current successor the Pelletron could deliver more current by ...
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### Why, precisely, do three elements in a row (iron, cobalt, nickel) show ferromagnetism, but not the elements below them on the table?

The same is true for Neodymium, Promethium and Samarium, right? But like the first three, they are consecutive, not above and below each other. Somehow only elements in those two ferromagnetic 'hot-...
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### 2 equal charged particles are fastened to the two ends of a spring of spring constant 100 N/m and natural length 10 cm. . find the extension

(assume extension to be small) I have the answer and I know how to solve this .we're taking equilibrium of each particle at a time and equating kx= kqq/r² but I don't understand how the x is the full ...
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### If we can think of free will as an illusion, why can't we do the same with gravity? [on hold]

I think the problem with gravity has to do with our limited understanding of what energy really is. There is no reason that a graviton would have to exist. Maybe in the current mathematical framework, ...
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### Hawking radiation of massive scalar field

There are some calculations about Hawking radiation (expectation value of particle number operator, 2-point function, stress-energy tensor, and so on.), which are often done under the condition that ...
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### Velocity of a spool of string being pulled

The question given is In working this, I assumed that it was in pure rolling motion and as such friction was negligible. Working through, I found \$ ΔKE = \frac{1}{2}Iω^2 = \frac{1}{2}I\frac{v^2}{R^2}...
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### Symmetry propertis of the 3-J symbol from the symmetry properties of CG coefficient

I use the relation between 3-J symbol and the CG coefficient. But using the symmetry propertis of CG coeficient, I am not able to arrive at the symmetry properties of 3-J symbol normally used. Even ...
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### A problem in newtonian physics

Hello! I have solved problem 89 using analytic way, i.e, the length of the string will always be constant, so repeated substitutions and differentiating will make way(i got answer as "c"). But is ...
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### How do I find the equivalent capacitance between A and B [on hold]

How do I find the equivalent capacitance between A and B
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### How does the collision between two atoms work?

Considering the quantum mechanical model for an atom, what exactly happens when two atoms (say, two Ca2+ ions in a Brownian motion) collide with each other? As I know, this collision is not like a ...