# All Questions

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### How does a wheelie work?

So I've been trying to create a mathematical model for an electric motorcycle and began to wonder about the maximum possible torque that could be supplied to the rear driven wheel without having the ...
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### What exactly is NASA's proposed mechanism for “propellantless” “EM Drive” propulsion? [duplicate]

Of course, this question runs perilously close to this site's prohibition against discussing non-mainstream physics. However, the accepted answer in meta about what is acceptable and what is not ...
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### Does the force of releasing the latch of a spring-latch contraption affects the force generated by the spring?

There is this contraption in my class, where a rod is attached to a latch and a spring. By pulling the latch back behind a piece of metal, the latch is secured, the rod if pulled back and the spring ...
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### Force of Gravitational attraction between the ring and the sphere? [on hold]

Let us say a uniform ring of mass m and radius 3a is kept above a sphere of mass M and radius 3a at a distance of 4a such that line joining the centres of ring and sphere is perpendicular to the plane ...
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### Bubbly Flow and two phase flow modelling

When we pour vine or some other drink why bubbles move upward and what law they follow or what is the law that describe that motion? One other question If we consider a simple two phase flow between ...
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### Any one know deep detail about artificial muscle from fishing line? (IYPT) [on hold]

Well, I have conducted some experiment about this IYPT problem, not for the IYPT but as a TYPT which is held in Thailand for Thai high school student. I have read all the references(full text, graphs, ...
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### Why does a reaction force still allow the thing exerting the force to move?

A $5$ kg and a $10$ kg box are touching each other. A $45N$ horizontal force is applied to the $5$kg box in order to accelerate both boxes across the floor. Ignore friction forces and ...
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### How to explain the following concepts regarding the behavior of the universe at early stages. [on hold]

Hello fellow physicists I'm working on a long question and having a little bit of trouble. "Discuss each of the following concepts in the context of theories of the early Universe and its ...
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### How to calculate angle subtended on the sky given the distances [on hold]

Here is my question: The Earth orbits the Sun with a semi-major axis of 1 AU. The Sun has a radius of 700,000 km. Use these numbers to calculate the angle subtended on the sky by the Sun, as ...
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### Do electric sinusidal waves rotate 45 degrees too when light pass through a hollow cylindrical magnet or just the sinusoidal magnetic waves do that?

Do electric sinusoidal waves rotate 45 degrees too when light pass through a hollow cylindrical magnet or just the sinusoidal magnetic waves do that?
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### Advantages of having a first class system and possibility of transforming a system into a first class one

I have two questions regarding first class systems. What are the advantages of having a first class Hamiltonian (a Hamiltonian whose all constraints are first class) in a theory or having a first ...
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### Question from radiation physic, photons per second [on hold]

A beam of red light, wavelength 700nm, has 3X10^6 photons per second and when incident on a thermophile is able to maintain an increase in temperature of 0.2K. How many photons per second is required ...
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### Meissner effect and levitation

In a field less than critical field, decreasing the temperature below the critical temperature will eliminate the magnetic field inside a superconductor and increase the magnetic field around it. ...
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### Electric current notation

Depending on the source, I sometimes read $\frac{\delta q}{dt}$ , $\frac{dq}{dt}$ or even $\frac{\delta q}{\delta t}$ (rare) Wich one is the correct notation ? In theory we are to know if a ...
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### Determining the components of the force on a curved surface due to pressure

I have a cross section of a half-tube with a pressure gradient across it causing a force outwards. I am attempting to extract the vertical component (in relation to diagram) of the force on this ...
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### Orbital angular momentum of electrons

In a QM class, to study the hydrogen atom, we started by defining the Hamiltonian $H$ for a central potential, then made an orbital angular momentum operator appear as part of $H$, then down the line ...
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### AC current through induction on both sides

When you rotate a coil in a magnetic field one side cuts downwards the other cuts upwards. When you move a piece of wire up and down you get an AC current or pdf which you can see on a voltmeter as ...
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### Point splitting technique in Pesking and Schroeder

One of the cornerstones of point splitting technique of calculating chiral anomaly (Peskin and Schroeder 19.1, p.655) is a symmetric limit $\epsilon \rightarrow 0$. And this is the point that I don't ...
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### Light - What is it? [on hold]

First of all, please excuse my bad english. I am from Germany. I am interested in, what light really is. I know, there is a range in the spectrum, and this range is visible. This is defined as light. ...
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### entanglement status of late hawking radiation in AMPS thought experiment

My first question! Can anyone explain to me why the late hawking radiation MUST be entangled with earlier Hawking radiation in order to ensure unitarity of the S-matrix? It seems to me the whole AMPS ...
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### Reltion between states of self stress and zero modes? [on hold]

I am trying to understand this paper: http://arxiv.org/abs/1308.0554 . What I do not understand is how he number of states of self stress enhances the number of zero modes. They write this in eqn. 1. ...
265 views

### What does the accelerating expansion of the universe mean about its end?

So we know that the expansion of the universe is accelerating.When we thought that the expansion of the universe was slowing down,we predicted that it would reach a plateaux and maybe reverse the ...
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### Do the same equations of motion imply the same Lagrangians?

If two Lagrangian (densities) $\mathcal{L}$ give the same equations of motion, are they equivalent?
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### The amount of potential energy at the height of h [duplicate]

When we lift an object upwards with a constant velocity for a distance of $h$ the work that we've done is $mgh$ and the work done by the force of gravity is $-mgh$. So the net work on the object is ...
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### Question on Thermodynamics andThe bigbang theory [on hold]

The third law of Thermodynamics states that 'the entropy of a perfectly crystalline substance at absolute zero is null' When you consider the state of the universe before the bigbang which holds ...
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### Colliding black holes to an outside observer

We know that a particle approaching an event horizon will appear to an outside observer to slow down and never cross the horizon. What is observed by an outside observer when a singularity ...
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### Blackbody cavity relationship between energy of oscillators and EM radiation?

This question is based on Planck's view of blackbody radiation in a cavity. Here is a quote from here: ...where $< E >$ is the average energy of the oscillators present on the walls of the ...
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### What does it mean for a particle to have spin of 2? [duplicate]

When I first started to study quantum mechanics, my physics text book told that particles have spin of either 1/2 or -1/2. Then I recently read an article saying that gravitons are expected to be ...
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### Which book do you recommend to start studying physics for a person who doesn't have any special knowledge about physics? [duplicate]

I'm studying bachelor of Mathematics in University but I'm interested in physics too. All of my knowledge about physics is what I studied in school and also the courses general physics $I$ and general ...
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### How large should an optically large fusion reactor be?

This is just theoretical consideration. I expect that size of the reactor would be impractical to build on Earth, but I'm interested how much. EDIT: perhaps it could be good for some huge spacecraft ...
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### Is time dilation based on the formula for period of a pendulum?

The theory Albert Einstein put forward about special relativity mentions a possibility for time dilation, in which he states gravity has a considerable effect on time. And in high school physics we ...
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### How to get these SUVAT formulae?

When learning kinematics, I have learned these three formulae $s=1/2(u+v)t$ $s=ut+1/2at^2$ $v^2=u^2+2as$ s = displacement u = initial velocity v = final velocity t = time a = acceleration My ...
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### Does the time ordering operator have a rigorous definition?

In quantum field theory, the time ordering operator (TOO) appears in the formal expressions for the scattering amplitudes. It acts upon a product of operators that each depends on time, and returns ...
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### Why does the mass of an object on a frictionless surface matter?

I'm given a physics problem about a climber hanging over a cliff and attached by rope to a rock on level, frictionless ice. The goal is to find the acceleration of the pair. After working out the net ...
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### Poisson brackets in curved spacetime

The time evolution of any field $\phi$ is given in terms of the Poisson bracket with the Hamiltonian, $$\frac{\partial\phi}{\partial t} = \{\phi, H\}.$$ How does this relation change in curved ...
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### What is the amount of charge density in a metal wire for the electron's which are involved in current flow?

According to the question Why is charge = nALe n is the charge density (number of electrons per unit volume). Which of the electrons in a metal wire are involved in current flow? Only the unbounded to ...
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### Validity of Press-Schechter formalism at the end of recombination

I was wondering if Press-Schechter formalism on Spherical collapse is applicable sometime at the end of recombination, i.e. the photons have started to free stream but the free streaming length is ...
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### What are the evidences that support the EmDrive? [duplicate]

Firstly, I'd like to point that I’m from Brazil, so forgive me for any errors in my writing. Today, i found myself reading a journal who said that NASA is closer to detecting Warp Bubbles predicted ...
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### Any tips on evaluating Riemann tensor?

I am calculating the Riemann tensor for the Schwarzschild solution. I've calculated all 9 non-vanishing Christoffel symbols already. Now I need to evaluate the Riemann tensor and I find no easy way to ...
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### Lie derivative vs. covariant derivative in the context of Killing vectors

Let me start by saying that I understand the definitions of the Lie and covariant derivatives, and their fundamental differences (at least I think I do). However, when learning about Killing vectors I ...
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### A different interpretation of $E=mc^2$ but no idea what it might mean [on hold]

I wanted $E=mc^2$ to look like an 'inverse square' sort of a formula. So this is what I derived: $E=mc^2$, so; $m=E/c^2$, assuming $E=E_1E_2$ (I am aware that when you decompose energy into two ...