3
votes
2answers
73 views

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 ...
2
votes
0answers
88 views

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 ...
1
vote
0answers
9 views

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 ...
0
votes
1answer
58 views

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 ...
1
vote
1answer
14 views

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 ...
-2
votes
0answers
31 views

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, ...
1
vote
2answers
83 views

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 ...
0
votes
0answers
27 views

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 ...
-1
votes
0answers
31 views

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 ...
1
vote
1answer
11 views

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?
1
vote
0answers
21 views

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 ...
-2
votes
0answers
12 views

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 ...
1
vote
0answers
18 views

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. ...
1
vote
1answer
44 views

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 ...
1
vote
1answer
40 views

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 ...
4
votes
1answer
41 views

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 ...
1
vote
0answers
12 views

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 ...
3
votes
1answer
51 views

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 ...
-4
votes
1answer
48 views

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. ...
1
vote
2answers
37 views

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 ...
-1
votes
0answers
11 views

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. ...
3
votes
3answers
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 ...
1
vote
2answers
90 views

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?
1
vote
4answers
49 views

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 ...
0
votes
0answers
26 views

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 ...
1
vote
0answers
31 views

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 ...
1
vote
0answers
25 views

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 ...
5
votes
1answer
572 views

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 ...
1
vote
0answers
10 views

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 ...
2
votes
0answers
29 views

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 ...
1
vote
2answers
70 views

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 ...
1
vote
2answers
38 views

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 ...
2
votes
2answers
56 views

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 ...
1
vote
0answers
56 views

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 ...
1
vote
0answers
24 views

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 ...
1
vote
1answer
33 views

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 ...
2
votes
0answers
13 views

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 ...
1
vote
0answers
26 views

Confusion about Shankar 1D Scattering in a Single-Step Potential

Section 5.4 of Shankar studies the problem of a single-step potential where $$V(x) = \begin{cases}0&x<0\\V_0 &x>0\end{cases}.$$ It was shown that a incident Gaussian packet $$\psi_I(x,0) ...
1
vote
1answer
15 views

Motion in a central field in Landau Mechanics

What does this mean when E=U_eff? I don't think this means the first term in E is zero. I don't understand the sentence ' This is a cubic equation for cos(theta)'
2
votes
1answer
52 views

Why can liquid mercury sit on salt?

A video shows liquid mercury sitting on top of table salt. Even though the mercury is much denser, it does not sink to the bottom. Why does this happen? Is the mercury just so thick that it is like ...
4
votes
1answer
40 views

Optimal size for a bike tire

While I was riding my bike funnily enough I wondered if there was an optimal size for a bike wheel. So I suppose that is my question, What is the optimal size for a bike wheel? If the bike accelarates ...
3
votes
1answer
34 views

Is there an algorithm for N body simulations in General Relativity [duplicate]

I am new to general relativity but have a background in computer science. Why is it so hard to do n-body simulations in GR? For example, there could be a program which takes the properties (mass, ...
2
votes
1answer
46 views

Is there a fundamental relationship between Coulomb's law and Newton's Law of Gravitation? [duplicate]

It seems like the two equations are identical indicates that there is something more going on that unites them both, maybe in a cool way or something. Why are these two laws so similar?
2
votes
1answer
55 views

Ehrenfest's Theorem “contradiction”?

Ehrenfest tells us that for $\hat{p}$ $$\partial_t \langle p \rangle = \langle -\partial_x V \rangle$$ I also understand the basic steps in deriving this result directly by taking the time ...
0
votes
0answers
46 views

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 ...
2
votes
4answers
76 views

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 ...
5
votes
2answers
76 views

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 ...
-4
votes
2answers
73 views

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 ...
5
votes
1answer
78 views

Basic question about superspace, Grassmann numbers and world sheet supersymmetry

So, I'm trying to read the section on superspace from the book on string theory by Becker, Becker and Schwarz, and I realized that I've been stuck on something simple for a while. Some relevant ...
2
votes
2answers
57 views

Decay of matter

I was watching Stephen Hawking's documentary and in there he explained how he realized why black holes eventually disintegrate: ...

15 30 50 per page