# All Questions

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### Why is it useful to study spatially inhomogeneous spacetimes? [on hold]

There are many scholars who have dedicated their efforts towards finding solutions to the Einstein Field Equations that are spatially inhomogeneous. Current cosmological observations indicate that on ...
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### What do you actually mean by DC voltage again and AC voltage gain? [migrated]

What do you actually mean by DC voltage gain and AC voltage gain? I know their formulas but I do not understand what they actually are? That is, basically I do not understand what is their ...
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### What do you actually mean by DC current gain and a AC current gain in transistors [duplicate]

What do you actually mean by DC current again and AC current gain? I know there for formulas but I did not understand what actually they are , what is the significance. So basically I do not ...
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### How actually sign reversal of output signal and the input signal takes place in case of amplification of transistor? [migrated]

How actually sign reversal of output signal and the input signal takes place in case of amplification of transistor? I am a class 12 student so make me understand accordingly.
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### How do we know that supermassive black holes formed in the early universe?

Could they have existed from the very beginning? Could they be leftover fragments of the cosmic egg?
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### Why does small change in Base current causes a larger change in collector current in case of transistors? [on hold]

Why does small change in the base current causes larger change in the collector current. That is say if I increase the base current buy a small amount why the collector current increases by a very ...
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### In causal dynamical triangulation, what equation(s) give the distribution of angles of the triangles?

I know very little about this topic, but going on what I learned in my one semester of QM, there has to be some Schrodinger-like equation they are using to get the distribution of angles of triangles ...
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### What is beam confinement?

In the context of the propagation of an electromagnetic wave and optical vortex solitons, I came across the term "beam confinement". Particularly, beam confinement requires the amplitude of the ...
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### Mutual or same set of eigenfunctions if two operators commute

If two operators commute, do they have "a mutual set of eigenfunctions", or "the same set of eigenfunctions"? My quantum chemistry book uses these as if they are interchangeable, but they do not seem ...
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### Formulae for Elastic Hysteresis of rubber

Being a school student, I'm pretty sure we don't need to explicitly learn any formulae helping us evaluate the energy dissipated by mechanical internal energy in a hysteresis loop. But I was wondering ...
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### To what height does the ball rebound and what is the velocity of the struck block after impact? [on hold]

Here is the problem. I have tried it four different times, and I will show my most recent work. [![enter image description here][3]][3]
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### What happens if an object stops orbiting the sun? [on hold]

What would happen if an object (like a space ship) was to stop its orbit around the sun. would time slow down with that object (the same way time sped up for the characters in that great film ...
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### Air flow speed and penetration abilities

As I understand, two fans with the same diameter and with the same rotor speed(usually measured in RPM) can have different air pressure(usually measured in mmH2O) and air speed(usually measured in ...
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### Modeling Fluid Flow

I could really use some assistance. How would I go about trying to use Maple in order to model flow of a fluid through objects? For a rough example, such as entering an enterance of a maze and exiting ...
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### General Solution to Linear Schrodinger equation

I am trying to find a solution to $$\displaystyle \left[-\frac{1}{2}\nabla^2 - \frac{2}{r} + C(r)\right]\phi(r) = E\phi(r)$$ where $C(r)$ is a known function of r. I am just looking for some help on ...
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### How bicycle gear works?

Based on the concept of the moment of inertia, my book gives the following relationship between the size of gear and amount of torque supplied to the drive wheel: Because it [outermost gear] had ...
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### Volume integral of pressure interpretation

If you have a fluid with with pressure field $p$ what is the physical significance of the the volume integral of the pressure $\int_V p dV$?
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### Pulley Friction

Why is the frictional force of a rotating pulley 0? For example, consider two masses and a string that links those masses passing through a pulley. Why do we not account for the frictional force of ...
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### Matter and dark energy

So dark energy is the biggest thing in the universe, it keeps accelerating the expansion. But it's a constant so at some point there was a matter - dark energy equivalence. My question is how can you ...
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### why don't large magents repel themselves

This question may have been asked before so I apologize in advance if it has. I've been playing with some magnetic buckyballs I got recently (highly recommend getting some if you dont have any) and ...
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### Am I exerting force when i am holding something? [duplicate]

So theoretically I found out that I am NOT exerting any force while I am holding something! Like Sir Isaac Newton said F=ma So technically it is true that when I hold something I am resisting it's ...
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### Rotation with axle with unequally sized wheels

https://youtu.be/hwgjMPaKMiA In the above video, I have an assembly of two wheels of unequal radii connected via an axle. Now when we apply an external force anywhere on the axle, we see something ...
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### Does force get divided among things?

If I exert a force to more than one thing at a time, will the force be divided among them or all of them will feel the same force? I mean imagine that I have tied up 3 bags of same mass with A rope ...
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### Can one measure experimentally the average distance between the two electrons in the excited state of helium in the singlet versus triplet states?

$\frac{1}{\lvert\ \vec(r_{1})-\vec(r_{2})\rvert}$ is a hermitian operator so I am supposing there could potentially be an observable with this which might be experimentally measured. We can measure ...
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### Why do objects traveling faster appear shortened rather than elongated?

I'm sure this seems like a stupid question but I hope someone can try to explain it as if to a child... This relates to Lorentz contraction. Why do objects in motion relative to the observer not get ...
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### Why does a top spin so well?

I know that the force of friction does not depend on surface area, but then why does a top spin better on a point rather than a region of larger surface area. For example, if I spin a top on it's ...
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### Is a $SU(2)$ supergauge theory really a $SU(2)$ gauge theory?

Consider $SU(2)$ supergauge theory with $A$, a doublet of two chiral superfields in the fundamental representation. $$A= \begin{pmatrix} \Phi_1\\ \Phi_2 \end{pmatrix}$$ where $\Phi_1$ and $\Phi_2$ ...
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### Criticality in BCS Theory

Can someone provide me with a pedagogical introduction into the role of criticality in BCS theory? The QCD condensate is due to strong coupling. The BCS condensation involves only weak coupling - ...
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### How to keep a helium balloon between 1 to 5 meters above ground? (without it being tied)

I understand that helium balloons rise because their density is less than air, so they can rise up to a point where the air surrounding it has the same weight as the balloon. I was thinking to fill it ...
I am confused between Green's function and vertex function in field theory. Can someone please explain the difference between the two in context ${\lambda} {\phi}^4$ theory?
Using the Friedmann metric I've been trying to calculate how long the universe was radiation energy dominated. I've reduced the metric to: $c.dt$ = $a(t).dr$ where a is the scale factor. I can work ...