# All Questions

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### Electrostatics kinematics equations (Coulombs Law)

If there are two similarly charged particles $Q$ and $q$ of masses $M$ and $m$, a Coulomb force is applied on each one (action-reaction). Suppose that $Q$ is stationary and we are shooting $q$ with an ...
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### Why does a star beyond a certain mass limit (Chandrasekhar limit) only become a black hole?

Why does a star beyond a certain mass limit (Chandrasekhar limit) only become a black hole? A star is first made of hydrogen, it undergoes nuclear fussion reaction combining into helium and ...
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### The effect of high frequency AC current in a fresh corpse [closed]

Given two AC currents of the same intensity, eg 20 mA, but different frequency, 60 Hz and 20MHz, which one would be more likely to reach the heart ? I've read about the skin effect and how current ...
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### How to plot $(x,y)$ coordinate of projectile motion (with air-resistance)? [closed]

I am trying to plot a graph to show the difference in projectile motion when it has air-resistance and when it doesn't have air-resistance. I set the mass, $v_0$ and $\theta$ as constant I can plot ...
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### Understanding the operation of Thomas precession

How can we physically understand the operation of Thomas Precession? This modifies the effective energy of coupling between the spin and the orbital angular momentum of the electron by an extra factor ...
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### Is this a valid proof that the four-current is conserved?

The four-current of a particle moving along a worldine $X^\nu(s)$ is defined as $$j^\mu(x^\nu) = ec \int u^\mu(s)\, \delta^4(x^\nu - X^\nu(s)) \, ds$$ So here's my proof that this is conserved: ...
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### Continuum limit for solid mechanics

Is there a rigorous derivation of the limits for continuum properties in solid mechanics? For instance, the stress-strain relationship may be linear for large samples (the slope being the Young's ...
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### Why meet at the center of mass?

If two objects of different masses are held at a distance $d$ and then I let them go, they will meet at the center of mass of the particle system due to mutual gravitational attraction My question ...
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### What becomes of the massive clouds expelled from Quasars?

What becomes of the massive clouds expelled from Quasar jets? Do they simply disperse into intergalactic diffuse clouds? Or do they on occasion have enough gravity to hold together and or collapse? If ...
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### Help me understand virtual images versus real images [duplicate]

Simple question; Is a virtual image that is produced by a lens always - in front of or in back of the lens?
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### Doppler broadening: time scale to frequency scale

Consider a cell containing Rubidium and enlight it with a laser. Connect the system with an oscilloscope and give a triangular wave as input (so you can know when the Rubidium is resonant). This is ...
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### Transportation using disintegration [duplicate]

Is it physically possible to have one device, that will scan one object atom by atom and record it to some computer file and then send it to some other machine that could use this blueprint to rebuild ...
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### Any examples of commensurable subgroups appearing in physics?

I am a mathematician. I am studying and working on Hecke pairs which I am going to give the related definitions in the following. But first let me explain what I am looking for to learn by asking this ...
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### Wald General Relativity, Chap 7.1

On page 166 of Wald's General Relativity book, he claims that the equation (7.1.20), $$0 = R^t{}_t + R^\phi{}_\phi = (\nabla_a t) R^a{}_b \xi^b + (\nabla_a \phi) R^a{}_b \psi^b,$$ yields (7.1.21), ...
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### Differentials in Spherical Shell - Maxwell Distribution

In explaining the Maxwell distribution of molecular speeds, my pchem textbook uses the following figure: We are basically trying to find the probability of having a particle with a speed $u$ between ...
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### Definition of Ohm in SI basic units in words

One way Wikipedia defines Ohm is (this is also teached in school): $$1\Omega =1{\dfrac {{\mbox{V}}}{{\mbox{A}}}}$$ They describe this definition in words, too: The ohm is defined as a resistance ...
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### Ideal gas law problems

I got really confused about real gases volume and ideal gas volume. Ideal gas molecules take up no space, if we put gas into a 2.4L water bottle, we know that all the gas will expand all over the ...
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### Given moment about a pin and a roller [closed]

sorry to ask, have not been a able to find a similar question to mine. For this problem, given moment $M_2$, distances $a$ and $b$ My reasoning is that we have two support reactions and points A and ...
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### Is it fair to judge this speedskating race by only 3 thousands of a second?

I'm reading this article: Zbigniew Brodka of Poland won the Olympic men's 1,500 meters speed skating title by just 0.003 seconds at the Adler Arena on Saturday. Brodka clocked one minute, ...