# All Questions

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### Why do people look for a field formalism for String Theory

String Theory was formulated originally from a perturbative description (using QM and replacing points by strings and doing path integral). Still, although QM has an upgrade to a field formalism -- ...
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### Was the firewall paradox resolved?

For quite sometime there has been a claim that the firewall paradox has been resolved (via lasers). For instance, http://global.ofweek.com/news/Lasers-to-solve-the-black-hole-information-paradox-9867 ...
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### Different results to a basic question ( Newton's law and perservation of momentum)

Trolley with mass of $m_0=1 \ kg$ is moving without friction on the railway track. It is raining so there is a constant mass flow of water $\Phi_m=0.1\ kg/s$. Constant force $F=0.1 \ N$ is ...
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### Relationship between Connection and Material Derivative

Suppose $D\subset \Bbb R^3$ contains a fluid and that $f : D\times \mathbb{R}\to \mathbb{R}$ is a time dependent function defined on the fluid region. In that case, the material derivative along a ...
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### String boundary conditions

I'm reading Polchinski and am confused about equation (1.3.13), $$\gamma_{\tau\sigma}\partial_\tau X^\mu-\gamma_{\tau\tau}\partial_\sigma X^\mu=0~~~~~\text{at}~~~~~\sigma=0,l.$$ It says that this ...
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### Are photons travelling in future? Are they lifespan 0 (or close to 0)?

We know that if we are going closer to speed of light, we are travelling in future. Since the photons are travelling at the speed of light, that means that they are travelling into future? Also, if ...
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### Does the coefficient of restitution matter in a collision?

Suppose two particles are colliding. Let initial velocities be $u_1$ and $u_2$ .Their final velocities are $v_1$ and $v_2$. Now $u_1$ and $u_2$ are known. By intuition I can understand that $v_1$ and ...
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### Normal force on varying mass

A cart filled with a mass $m_0$ of sand leaks it at a constant rate of $\mu \ \mathrm{kg/s}$ through a hole at the bottom of the cart. Suppose that the cart is moving on rails. Find the ...
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### Capacitance of air capacitor [on hold]

Hi I'm currently working on the following question (image below), any help is much appreciated!
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### Calculation of cable properties in parallel circuit

i am working on a simple program and need your help for a problem. I have a power grid circuit and between 2 nodes there are from 1 to many cables. But the program accepts only one cable between 2 ...
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### Why do stickers curl?

When peeling a sticker off its base, the immediate reaction is that it curls; why is this? I am having trouble finding an answer to this. Could it be that the glued side expands upon contact with the ...
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### A moving inclined plane with block problem

I'm trying to solve for a challenging problem : a moving inclined plane with a block I want to solve for the acceleration components for the block and the plane and the normal force acting on the ...
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### Why do I always hear remote train horn at night?

This is definitely not an illusion, as many people have the same experience. I have usually lived in places miles away from train stations, which makes it unlikely to hear any train horns during the ...
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### General definition of vector spinor and spin

I am looking for basic and exact definitions of fundamental physical consepts in graduate level. I reach this following definitions. Could you please help to improve these definitions. Spin: ...
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### what is the mean velocity of this? [on hold]

The Maximum velocity of a one dimensional incompressible fully developed viscous flow, between two fixed parallel plates, is 6 m/s. The Mean Velocity ( in m/s) of the flow is (A) 2 (B) 3 (C) ...
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### How do you obtain the fluctuation spectrum of a tubular membrane?

I am reading through a paper. A tubular membrane, submitted to tension $\sigma$ acting as a Lagrange multiplier to conserve area, fluctuates around a cylindrical shape of length L and radius R. ...
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### Why doesn't gravity bend everything equally?

If gravity is the curvature of spacetime it should bend everything equally. To clarify my point I would like you to imagine two scenarios. Think of a bird flying in the storm while the wind is blowing ...
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### Glauber's Formula

In the Cohen-Tannoudji Quantum Physics book, Complement BII, says: [...] two operators $A$ and $B$ with both commute with their commutator. An argument modeled on the preceding one shows that, if we ...
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### Is {299,792,458^2 * (6.67384*10^-11)} the number that can be considered a moment of inertia? [duplicate]

Whence x is a variable, and given E=mc^2 and F=Gm1m2/r^2 and E/F=x, is approximately 59,981,482,620,609,230,385,376,000,000 the number that the equation (mc^2/Gm1m2/r^2=x) reduces to?
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### Solar stills with Wine [on hold]

Is it possible to obtain more concentrated alcohol by using wine in a solar still instead of seawater? If yes, as I suspected, how does the concentration (of alcohol) of the distillate vary with ...
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### Why is it so hard to mix spaghetti with meatballs?

When I cook spaghetti with a sauce that includes meatballs or chopped vegetables, those stay in chunks and won't get uniformly mixed in the pasta. Is there a simple reason for that? Has it been ...
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### Orbital Speed and Centripetal Acceleration [on hold]

I need help in solving this A space station is in circular orbit around earth at an altitude of 5.0×10^5m. If the station makes one revolution every 95 min compute its; A. Orbital speed ...
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### Does the moment of inertia equal “the number attached to c (speed of light)” divided by “our most precise numeric value attached to G”? [duplicate]

Whence x is a variable, and when E=mc^2 and F=Gm1m2/r^2, and when E/F=x, can one reduce the equation mcc/gm1m2/r^2=x to a ratio of dividing the numeric value of the speed of light by our best ...
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### What measurement of time is so small that it qualifies as quantum?

Quantum mechanics (QM; also known as quantum physics, or quantum theory) is a branch of physics which deals with physical phenomena at nanoscopic scales where the action is on the order of the ...