# All Questions

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### How would a plane hitting a ball move (opposite of Pong)?

When a moving ball hits a stationary plane at an angle of incidence to the normal, it bounces away at the same angle (the angle of reflection), which is commonly understood. My question is 1) What ...
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### How to derive $r, \theta, \phi$ for the sperical coordinate gradient?

I'm trying to figure out how to get the gradient in spherical coordinates. I'm as far as the author writes in this answer: http://physics.stackexchange.com/a/78514 and I understand how and why to get ...
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### Will the day/night cycle change when the poles melt?

I want to know what will happen to day and night if poles melt i think it will change but I don't know why? ^_^
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### Total Energy of Inverted Pendulum

I have to find the total energy $V$ of an inverted pendulum (rod). The following parameters and their values are given: Mass $m$ and length $l$ of the pendulum $\theta$ as the angle of the ...
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### In an elastic collision, can we choose between cons. of energy and cons. of momentum?

Suppose we have two solid spheres with masses $m$ and $M$, respectively, and that $m$ is significantly smaller than $M$. The lighter sphere is placed directly on top of the heavier one, and the two ...
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### How does $p=fv$ hold when $v=0$ and an engine is working?

Standard high school equations: Work = Force x Distance Power = Work / Time = Force x Distance / Time = Force x Velocity When a space rocket takes ...
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### The structure of spacetime and entanglement [on hold]

how many ways to quantize gravity? Do they contradict each other? How to apply long-range entangment/Professor Wen Xiaogang's theory to virtual pairs
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### In aircraft design why are light materials preferred to heavy ones?

Especially given the relative cost between (say) steel and carbon composites. After all, I assume most fuel is consumed overcoming drag not accelerating mass. Once an aircraft reaches cruising speed ...
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### plasma and infinite acceleration

I am trying to set up a simulation of particles (they are not any particular kinds of particles, they are just points that repel each other). All the particles repel each other through the inverse ...
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### “Definition” of internal energy

Conversation of energy implies that if we have a thermally insulated system which goes from state 1 to state 2: $$\Delta E_{12}=E(2)-E(1)=\Delta W_{12}$$ and the 1st law of thermodynamics ...
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### Does it take more energy to open a door when applying force close to the hinge?

Assuming an ordinary hinged door (without any springs), would it take more energy to open it when applying force in the middle of the door (point b), rather than at the end of the door (point a), ...
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### Hysteresis and dissipation

Hysteretic phenomena are often linked to dissipation. When there is an hysteresis loop, the dissipated energy can usually be computed as the area of the cycle. For example, in ferromagnetic ...
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### Calculate bicycle efficiency power transmission?

How can I calculate the efficiency of power transmission from a person to a bicycle with standard flat pedals? How can I calculate the efficiency of power transmission from a person to a bicycle with ...
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### Can a hypothetical universe have more than 2 types of its dimensions?

Our universe is often described as having 3 space-like dimensions and 1 time-like dimension. Can hypothetical universe exist with more than space and time like dimensions? If so how would these ...
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### What would happen if a space ship may travel as quickly as the speed of light? [on hold]

Ofcouse, it is impossible. Beacuse of need of infinite energy to accelarate infinite mass... Bur if it might be possible...??? I did a fictional study as an amateur. I shared it, but i need feedback ...
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### How to interpret single photon interference when the two possible paths are different in length?

Here is my question. I struggle with the definition of single photon interference. Let’s assume we have a Michelson interferometer and the interference pattern we observe is a single photon result, ...
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### How does the second law of thermodynamics affect the efficiency heat-to-work conversion, at the molecular level?

I understand the second law of thermodynamics in terms of the improbability of the high--and-low-velocity particles of a gas to separate themselves so that at one moment they are at different sides of ...
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### Quantum state of a system after measurements with non-commutative operators

a) Assume two operators $A$ and $B$. 1) Assume $$[A,B]=0$$ and $$ψ= \sum c_n u_n ~~~~\text a~ wavefunction~ describing~ the~ state~ of~ the~ system$$ with $$Aψ=a_n u_n$$ $$Bψ=b_n u_n$$ If we ...
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### Another universe in the other side of a black hole? [duplicate]

A common notion is that there is another universe in the other side of a black hole, past the singularity point....is this true? does it have any scientific support?
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### How do magnets work?

I've read a classbook on the field theory (including EM): it perfectly describes quantitive patterns in EM-theory, but I have no luck understanding how and why it works. I mean, magnetic substances ...
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### Deriving Birkhoff's Theorem

I am trying to derive Birkhoff's theorem in GR as an exercise: a spherically symmetric gravitational field is static in the vacuum area. I managed to prove that $g_{00}$ is independent of t in the ...
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### What is the formula of power of an optical instrument?

I have searched and found that the power of a lens, with surrounding medium of refractive index $n$, is $n/f$ where $f$ is the focal length, the formula is $$n/f=(n'-n)/R1 + (n-n')/R2.$$ But in my ...
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### Is there any kind of material/magnet which do not attract any other magnet or metal [on hold]

Is there any kind of material/magnet which do not attract any other magnet or metal
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### How would I prove the gamma normalization relation without Clifford Algebra? [duplicate]

The equation in question is $$\left ( \gamma ^{\mu } \right )^{\dagger }=\gamma ^{0}\gamma ^{\mu }\gamma ^{0}$$
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### Birkhoff Theorem proof reference [on hold]

I am looking for a reference with a clear (and easy to understand) proof of Birkhoff's theorem in general relativity.
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### Zero volume at zero Kelvin

Why does the volume of a gas become zero at 0 Kelvin? Can a Bose Einstein condensate be considered as matter? (I mean the volume becomes zero)
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### Why do really powerful magnets have to be really big?

If you want to make a really powerful magnet, that can for example attract golf ball sized metal objects from 15 feet away, the magnet has to be really big, and really heavy. Why is this so? I don't ...
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### Schrödinger Equation [on hold]

Thank you for putting my question on hold. If you will allow me a few days, beyond this weekend, to adequately rephrase the question. I need the time to find a local physicst/math professor to aid ...
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### Can conservation of momentum be violated?

The law of the conservation of momentum has been established for hundred of years. Even in Quantum field theory every particle collision must be momentum-conserving if there is homogenity in space. ...
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### Question on derivation of Ward identity

I'm currently reading these notes about the Ward identity (pages 259 - 261). I will repeat some of the steps to make the question self-contained. Let us consider a local transformation on the field ...
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### Historical Survey of Statistical Mechanics

Statistical mechanics is a subject with a particularly rich history. I think of the early debates of Boltzmann and Loschmidt, the rather confusing differences between the approaches of Gibbs and ...

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