# All Questions

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### How to find the height where the collision of two balls occurs?

There was this question that got deleted (for whatever reason): Ball A is dropped from the top of a building of height h at the same instant that ball B is thrown vertically upward from the ...
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### Can electricity flow through vacuum?

People say yes, and give a wonderful example of vacuum tubes, CRTs. But can we really say that vacuum (..as in space) is a good conductor of electricity in a very basic sense?
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### How to show time reversal symmetry is not break in the tight binding Hamiltonian for the honeycomb lattice?

The Hamiltonian of the honeycomb lattice is $$H=\sum_{k\sigma}t(k) a_{k\sigma}^\dagger b_{k\sigma}+h.c$$ Where $t(-k)=t^*(k)$. If we do a time reversal transformation(according the answer to this ...
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### hiding joint lines of plain mirrors

For some experiment I took two rectangular pieces of front surface mirrors, and joined it at 30 degree. However, the joint line is visible clearly, how do I hide it ? I guess I need to apply some ...
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### What is the idea of canonical quantization?

From what I understand, canonical quantization of a classical theory consists of replacing the observables by abstract operators, of which only the commutation rules, which have to correspond to the ...
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### Does Average Lifetime even mean anything?

So today I was trying to derive an expression for the number of radioactive atoms remaining after a time $t$ if I began with $N_0$ atoms in total. At first I tried to assume that they had an average ...
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### Around fixed point of renormalization group

In the general formulation of renormalization group in "statistical mechanics" by P.K.Pathria, each point in parameter space is represented by a vector $\vec{K}$ and the transformed vector would be ...
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### Efficiently compute the bases functions of the Signal

Let there be 4 1D signals such that \begin{cases} x(t)=4\sin(10\pi t) \\ y(t)=8\cos(20\pi t) \\ z(t)=16\sin(30\pi t) \\ m(t)=x(t) +y(t)-z(t) \end{cases} Is there a way to compute ...
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### Acceleration of an oscillating object in a frame of reference that is itself rotating!

I have been reading a paper and due to my limited knowledge of Physics, I can't move ahead. Sorry I do not know latex so, I will snip the paper and paste it here. So here goes it..... I think ...
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### Equivalence of gauss's law for surface and volume distribution of charge

I have a problem about Equivalence of Gauss's law of surface and volume charge distribution. Can any one tell me about the equation which relate the surface and volume distribution of charge using ...
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### String theory $bc$ system CFT

$bc$ CFT is an example of free-field CFTs. action is $$S = \frac{1}{2\pi} \int d^2 z b \bar{\partial} c$$ How can we obtain equation of motion? Polchinski-volume 1-page 50 writes that ...
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### Is the concept of infinity valid in physics, beyond its convenience?

Is infinity a meaningful concept in physics, apart from making some of the maths easier? Especially since reality seems to be discrete.
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### can we calculate length and direction of radio waves that how far radio waves are coming and from which direction…?plz tell if anyone know

I am asking this for programming purpose.
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### Double Pipe Heat Exchanger Flow Hot Cold

Based on sources I referred, I got to know that a double pipe heat exchanger operates in a way where hot water flows in a pipe and comes out as cold water and cold water flows into another pipe and ...
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### What will happen if Sun makes a trivial change in position?

What will happen if Sun makes a trivial (negligible) change in position. Will it be disastrous? Or, It will not matter?
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### If my particle is a surfer on a wave and speed of wave is $c$, How can I add “the speed of particle” in relavity?

I have 3 particles which have different masses (0, 5 and 10). I have also a wave with the speed of $c$. My particles are also vibrating in $y$ axis (according to direction of waves) because of peaks. ...
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### Is Brent's accelaration zero at any point? If so, where? [on hold]

Brent is jumping on a trampoline.He leaps up into the air,and falls back again.
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### Is there a way to photograph the very edge of black hole?

According to Professor Stephen Hawking, black holes emit radiation, Hawking radiation. The bigger the black hole, the fainter the radiation. That means, I personally believe, if we built large enough ...
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### Is there a difference between Electric and Electrostatic Field?

Is there a difference between Electric and Electrostatic Field? All I know is that they both represented with same law suppose we have a Charge placed at the Origin: ...
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### When and who by was the earliest definition of speed?

Speed is defined to be distance divided by time; when and who by was this definition first put forward? The obvious guess would be for Galileo in early modern physics; is this right, and can it be ...
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### Induction Oscillation [on hold]

A bar (m=100g, l=10cm) is moving in a constant magnetic field $B$ as shown above. The resistor $R$ is $1\Omega$. The bar is attached to a spring with spring constant $10N/m$, can you derive ...
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### Why do rings form on microwaved CD/DVDs?

Here's a microwaved CD. As you can see, the disc has been destroyed, but there are very clear circular rings bridged with lines. Why are rings formed when microwaving discs?
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### space contraction near lightspeed

I read a lot of the Time-Life Science books when I was a kid (well, mostly the pictures and captions). One of the things that always stuck in my mind was an explanation of the twin paradox, using ...
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### Why is the rotation not zero and the divergence zero in the figures below? [migrated]

Why is the rotation not zero and the divergence zero in figure 1 and figure 2 below? Figure 1 Figure 2
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### Interesting approach to Kepler problem

I won't go into the explanation of this idea, because it is explained in this blog post. In this paper, which was featured on John Baez's blog, $\frac {dt}{d\lambda}$ is given as $\frac r V$, where ...
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### Relationship of the volume of a plumbing system to pressure

How does the volume of water a plumbing system holds vary with water pressure? I know the ratio would include the modulus of elasticity of the plumbing material, the total surface area of the plumbing ...
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### Weightless person in a cart on a rail [on hold]

In a problem I'm trying to solve I've having a conflict between my understanding and the proposed solution of the problem. A cart ($m=35kg$) is at rest in the start of a friction-less ("covered with ...
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### Geophysics: Dynamic form factor and Equipotential surfaces [on hold]

I've got the following problem which I need help with. I'm used to calculating coefficients when the problem is about ellipsoids and first order approximations. But when it comes to spheres and ...
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### Propagation of incertitude - specific example [duplicate]

I am setting up an experiment, where I know the force and the incertitude (delta) of that force. However, I need to graph the logarithm of that force with the incertitude of the log. Can someone help ...
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### A question about genus one string amplitude

In BCOV's paper http://arxiv.org/abs/hep-th/9309140 the genus one string amplitude of a Calabi-Yau 3-fold was explained in the B-model as the Ray-Singer torsion (there is a similar discussion in the ...
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### Why is the heat transferring best from within the tube within a tube-heat exchanger

If you've got gas and cooler fluid, you'd run the gas through the tubes, because the gas has a lower h-value. Inside a tube heat-exchanger it is said that the best heat transfer is from within the ...
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### Moment of Inertia Calculation [on hold]

A uniform disc has centre O, radius a and mass 2m. It is free to rotate in a vertical plane about a horizontal axis through O. A particle P of mass m is placed on the highest point of the rough edge ...
A homogenous beam $OA$ with the length $4a$ and the mass $m$ can rotate in the vertical plane along a horizontal axis through $O$ and is kept in equilibrium by a wire running from the points A and C ...