# All Questions

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### Question regarding the angular velocity of a ball rolling off of a sphere

Here is the actual problem in its entirety: A uniform ball with radius r is rolling down from the top of a fixed sphere with the radius R. Its initial velocity is negligibly small. What will be the ...
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### Masses connected with a string, one mass in unifom circular motion

A light inextensible string of length $a$ is threaded through a fixed smooth ring $R$. One end of the string is attached to a particle $A$ of mass $3m$. The other end of the string is attached to a ...
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### Does the fact a charge experiences a perpendicular force when moving perpendicular to a magnetic field have anything to do with EM waves?

Does the fact a charge accelerates perpendicular to a uniform magnetic field when moving perpendicular to it have anything to do with the form of an EM wave? I thought this might link because the ...
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### How to count the number of microstates in this example and in general

Suppose we have $3$ energy levels $E_1,E_2,E_3$, that we need to fill up with $3$ electrons. Each of these Energy levels can be filled with two electrons - spin up and spin down. How does one ...
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### The Reaction at A is ____ kN and at B is ____ kN [closed]

The Reaction at A is ____ kN and at B is ____ kN.
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### "Active quantum fluctuations" due to the quantization of the gravitational field?

This is a follow-up to the question In what sense is the word quantum fluctuation used here? In arXiv:0710.3787 it is stated on page 7 that there are "active fluctuations in the intrinsic ...
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### Is a scalar obtained by metric contraction invariant under Weyl transformation?

Under a Weyl transformation $g_{\mu\nu}(x)\rightarrow e^{2\omega(x)}g_{\mu\nu}(x)$ So a scalar $X_\mu X^{\mu}$ should transform as following: X_\mu X^{\mu}=g_{\mu\nu}X^\nu X^{\mu}\rightarrow e^{2\...
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### Can we bend light ray into any closed loop?

Suppose we have a medium with varying refractive index and a source of light inside that medium emmiting rays. Then is it possible to bend the ray into any closed loop? As the medium has varying ...
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### How to understand the time reversal symmetry of position operator?

How to understand the fact that position operator is symmetric under time reversal? I can visualize the momentum and magnetic field being odd under time reversal. Got the same doubt for Electric field ...
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### Rocket orbitting Moon [closed]

The Figure shows six paths by which a rocket orbiting a moon might move from point a to point b. Rank the paths according to the net work done on the rocket by the gravitational force from the moon, ...
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### What does Polchinki mean when he says identifying under involution?

I am reading chapter 7 of Polchinki's String Theory vol. 1 textbook and I am trying to understand why one obtains the cylinder (annulus) from the torus by identifying points under involution. To be ...
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### Is light a single wave when we refer to frequency?

When we say that light oscillates $n$ times (frequency) in a second do we mean that the same electromagnetic field travelling through space oscillates $n$ time's? As i have seen diagrams suggesting ...
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### How to generate a PDE from a discrete equation in a rice-pile like model?

I am reading Noise and dynamics of self-organized critical phenomena by Albert Díaz-Guilera Here, on an extension of the rice-pile model by Bak et al demonstrating self-organized criticality. Equation ...
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### Particle- and wave-like properties

Is wave-particle duality a real concept or a pedagogical tool? In a less opinion-based way: what are particle properties and wave properties of a particle (that we speak of particle properties of ...
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### How do I prove this integral solution in terms of modified Bessel function?

I came across this equation while studying about Dark Matter relic density but I can't seem to prove it. In the early universe, the number density of dark matter in the thermal bath is approximately ...
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### Why don't wave packets disperse? [closed]

As the Uncertainty Principle suggests that $\Delta x\Delta p\geq h/4\pi$, a particle must have different velocities at any point in space within it's Gaussian distribution. Since wave functions ...
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### A wire that is electrically neutral but carries current?

The wire described in the title is proposed in Griffiths's book on electrodynamics: ... To begin with, I'd like to show you why there had to be such a thing as magnetism, given electrostatics and ...
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### What does "marginality" mean in the context of magnetically confined plasmas?

I am reading the paper-Self-organized criticality and the dynamics of near-marginal turbulent transport in magnetically confined fusion plasmas by Sanchez and Newman. Here, the term "marginality&...
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### Is general relativity required from quantum mechanics?

One of the paradoxes against quantum mechanics was claimed by Einstein during one of the Solvay conferences: a paradox called the Einstein box also explained in this question. The solution proposed by ...
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### Does pressure affect heat transfer between two mediums?

I'm a self-taught Coca-Cola connoisseur and try to use science to be able to have the best taste possible in every glass. I usually try to get the bottles as cool as possible before opening them. I've ...
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### How do you figure out where an object between two charged objects needs to be for the net electrostatic force acting on it to be 0? [closed]

In a question such as: There are three objects (A, B, C) in a line. A has a charge of +5 microCoulombs, B has a charge of +3 microCoulombs, and C has a charge of +2 microCoulombs. A and C are 7 cm ...
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### Cosmic inflation and how does it stop?

What conditions make out the probability for the Cosmic Inflation once began to stop. For instance quantum physics says that once the inflation started there was a probability say 80% for it to stop ...
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### What does it mean to apply a voltage bias on a superconductor when $V = IR$ and $R = 0$ for a superconductor?

I have been reading papers about superconducting nanowire single photon detectors, and in them the voltage bias the superconductor to close to $T_c$ so that a photon has enough energy to break the ...
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### $t=0$ is out of the domain of the equation of motion

At $t=0$, $Q_1$ is dropped under the influence of only the electrostatic force. I have solved the differential equation to find the equation of motion: and the distance of the 2 objects as a ...
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### What does it mean to take GR and QM "together"?

From the "Theory of Everything" Wikipedia article (first paragraph): Over the past few centuries, two theoretical frameworks have been developed that, together, most closely resemble a TOE. ...
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### Finding windspeed gave me negative time value

This is a homework question, I will make it as concise as possible: A 1-way plane trip is 2800km and the plane's airspeed is 790km/h. The plane flies with the wind on the way there, and against it on ...
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### Centrifugal Force & Rotating Frames

In Thornton & Marion's Classical Dynamics, the following relation is given for the rate of change of an objects position in the two coordinate systems (according to the picture shown at the bottom)...