All Questions

0
votes
4answers
33 views

How to find total force on an object?

If there are are a multitude of forces on an object, acting on it in different directions, how do you find the TOTAL force? I know you add up the x components and the y components individually, but ...
0
votes
0answers
21 views

maximun work in a reversible process

I often read maximum work done is by a reversible process. Shouldnt the more appropriate sensence should be maximum work done per unit of heat is maximum in a reversible process. ?
1
vote
0answers
34 views

Electric field at a point [on hold]

We consider a thin disk whose center is $O$ and radius is $R$, which has an electric surface charge density of $\sigma$. We want to find the expression of the electric field $\vec{E}$ at a point ...
1
vote
0answers
35 views

What does “nonlocal in time” mean?

I am studying MOND theory. At some point Milgrom says the theory needs to be nonlocal in time in order to enjoy Galilei invariance, but I don't understand the meaning. Which are the major ...
2
votes
0answers
19 views

Measure of interaction of two quarks and Casimir operators

Let's have two quarks, which refers to representations of $r_{1}$ and $r_{2}$ of color symmetry group. They create bounded state which refers to the representation $r$. There is a statement that ...
3
votes
1answer
77 views

Rigorous version of field Lagrangian

In Classical Mechanics the configuration of a system can be characterized by some point $s\in \mathbb{R}^n$ for some $n$. In particular, if it's a system of $k$ particles then $n = 3k$ and if there ...
1
vote
1answer
27 views

Wavefront sensor specifications Shack-Hartmann

Why is the dynamic range of Shack-Hartmann wave-front sensors (As seen in the first image below) quoted in wavelengths and not in angles? I thought that one of the most important aspects of the ...
-4
votes
0answers
18 views

Applied Mechanics Statics of a Rigid Body [on hold]

The I-beam $A-B$ shown below is supported by a cable at $B$ and a pin at $A$. The cable is inclined at an angle, $\theta = 23°$ to the longitudinal axis of the beam. The beam supports a load, $m$, of ...
4
votes
2answers
54 views

Spontaneous symmetry breaking to subspace not giving massless bosons

I'm currently trying to understand spontaneously broken in general and have stumbled upon a weird result which doesn't seem to correspond to my knowledge about broken gauge symmetries. Suppose we ...
0
votes
0answers
51 views

Heisenberg approach of Quantum Electrodynamics

I am reading the book of Gunar Kallen "Quantum Electrodynamics" and in the Chapter VI he study the Vacuum polarization. He computes the experimental observable current ...
7
votes
2answers
204 views

Is reflection instantaneous?

I was wondering while reading "On the Electrodynamics of moving bodies" by Albert Einstein (1905) (Translated to English). In the paper, he describes the time as being: by definition that the ...
0
votes
0answers
17 views

Direction of particle's vibration.

I have seen a question related to the the direction of the vibration when a wave moves. I see there two opposite picture that represents contradictory notion of getting the accurate direction of the ...
0
votes
0answers
29 views

Arrow of Time in Information transfer

I am writing a sci-fi script and need some legitimate theory to back up a central story element (so there's no real world application): Could there be a logically consistent theory supporting the ...
-1
votes
1answer
22 views

Is the probability density of a finite potential well independent of time or not? [on hold]

The probability of the first excited state of a finite potential well is time independent or not? Why?
1
vote
0answers
27 views

Is it possible to predict the weather like John Locke? [on hold]

In series 1 of the TV show Lost, the character John Locke seems to be able to predict the weather preternaturally well; he says, during a monsoon, that the rain will stop in a minute, and lo and ...
4
votes
2answers
117 views

Understanding the Jacobian Matrix

Taking the example of a two dimensional system, desribred by the following ODE's: \begin{align} \frac{dx_1}{dt}&=f_1(x_1,x_2)\\ \frac{dx_2}{dt}&=f_2(x_1,x_2) \end{align} The Jacobian Matrix ...
5
votes
0answers
49 views

Zamolodchikov's c-theorem paper

I am reading the 1986 paper [1] where Zamolodchikov proves the c-theorem and I would like to understand how equations (7a), (7b) and (8) are derived from the Callan-Symanzik equation. For ...
1
vote
1answer
36 views

The concept of torque among other unknowns

Someone asked me this question and I am going to try to ask it as 'proper' as possible. Imagine there is a door and it needs a mass on the floor at the edge of the door to keep it from swinging shut. ...
0
votes
0answers
29 views

How to accurately mark with arrows an electric circuit

As I understood, we choose to mark the current with arrows going from the plus pole to the minus pole (even though we know that in reality it is the contrary). As I'm looking at electric circuits, I ...
0
votes
1answer
21 views

Defects of smart glass (electrochromic glass)?

Smart glass is a type of glass that uses electrochromic mechanism to change it's light transmission properties in response to voltage, and it's made by sandwiching certain materials between two panes ...
0
votes
0answers
24 views

Can multiple lasers be concentrated in a circular area using some optical arrangements?

Can multiple lasers be concentrated in a circular area using some optical arrangements such as using a paraboloid surface? If yes, why do in high laser power systems do they use only 2D arrays of ...
2
votes
2answers
35 views

Is the coefficient of restitution frame independent and energy conservation?

In this question I am ignoring relativistic effects. The following statements I think are true: change in Kinetic energy is not invariant under change of frame Force is invariant under change in ...
3
votes
0answers
44 views

Weinberg QFT Chapter 5: gamma matrices consistency

Currently reading through Weinberg's QFT book (Vol. 1) [readable in parts here]. In his derivation of the causal Dirac field in Ch. 5, he chooses his gamma matrices as (5.4.17) \begin{align} ...
2
votes
1answer
207 views

Generalization of canonical commutation relation

The canonical commutation relation $$[x,p] = i\hbar$$ can be generalized to $$[p_i,F(\vec{x})] = -i\hbar\frac{\partial F(\vec{x})}{\partial x_i}, \ [x_i, F(\vec{p})] = i\hbar\frac{\partial ...
2
votes
1answer
61 views

A question regarding collisions

Let us consider a system of 2 identical spherical bodies connected by a massless string that is taut. If one body is placed at the origin then the other is placed at some coordinate $(x,y)$. The ...
1
vote
1answer
23 views

Johnson-Nyquist noise for resistive element with temperature gradient

In Nyquist's 1928 paper, Thermal agitation of electric charge in conductors, the voltage noise for a resistor is derived assuming a circuit in thermal equilibrium and one temperature. How does the ...
1
vote
0answers
38 views

Natural units in a parallel universe [on hold]

Taking ispiration from this question units and nature . The natural Plank units set the gravitational costant $G$, the planck costant $\hbar$, the speed of light $c$, the Boltzmann costant $k_B$ and ...
0
votes
1answer
30 views

How should I calculate uncertainty of measurement calculated as average of two measurements

I am measuring force with two channel transducer. Both channels (separately) of this transducer has been calibrated and I can calculate uncertainty of measurement for each of it. However I want to ...
2
votes
1answer
43 views

Fermion propagator decomposition

I've seen the following decomposition for the fermion propagator for a fermion with momenta $p-k$, and where both $p-k$ and $p$ have a mass of $m$: $$\frac{(\not p-\not k)+m}{(p-k)^2-m^2}\gamma_\mu= ...
-2
votes
0answers
30 views

Two masses attached by a spring (differential equation) [on hold]

Consider two masses $m_1$ and $m_2$, connected by a spring of spring constant $k$ and an uncompressed length $L$. The masses are on a frictionless surface. Initially, $m_2$ compresses the spring to $L ...
0
votes
1answer
46 views

Energy without temperature

If you know the entropy $S$ of your system. Is there a general way to calculate the internal energy $U$ of your system? So the entropy $S$ is the only thing I know of my system. I have no information ...
0
votes
1answer
35 views

2 vacuum pumps in parallel

I have a problem with a vacuum pump that cannot generate enough vacuum to grip an uneven item. There is a spare vacuum pump in parallel which I will turn on and run at the same time to try and get ...
0
votes
0answers
34 views

Is a model of helium atom without electron-electron repulsion solvable analytically?

Consider the following Hamiltonian: $$\hat H=-\frac{\hbar^2}{m_n}\nabla_n^2-\frac{\hbar^2}{m_e}\nabla_{e1}^2-\frac{\hbar^2}{m_e}\nabla_{e2}^2-\frac Q{|\vec r_{e1}-\vec r_n|}-\frac Q{|\vec r_{e2}-\vec ...
2
votes
1answer
69 views

What is the four-dimensional representation of the $SU(2)$ generators?

Recently, I have been learning about non-Abelian gauge field theory by myself. Thanks @ACuriousMind very much, as with his help, I have made some progress. I am trying to extend the Dirac field ...
0
votes
1answer
19 views

Bouyant force in air on hollow object

I understand that the buoyant force is equal to the weight of the air displaced by the volume of the object. I also know that if we had a perfectly solid cube or a hollow cube closed on all sides so ...
1
vote
0answers
41 views

My idea on parallel universes [on hold]

I have a question I've long wanted to ask somebody and I see no place better than here to do it! I am not a physicist in any way, Im just a big fan :) So, I have an idea of parallel universes in my ...
4
votes
1answer
285 views

Switching from sum to integral

I'm specifically asking about an equation in An Introduction to Quantum Field Theory, by Peskin and Schroeder. Example from page 374: $$\mathrm{Tr} \log (\partial^2+m^2) = \sum_k \log(-k^2+m^2)$$ ...
2
votes
1answer
43 views

The influence of the antenna height

I am working on a model of a transmitter. The transmitter is attached to the wheel of the vehicle and thus constantly changes it's height. In other words capacitance between antenna and ground is ...
5
votes
2answers
173 views

Poisson brackets of the Kepler Problem

For the hamiltonian of a particle of unit mass in a kepler potential: $$H = \frac{1}{2}\mathbf{p} \cdot \mathbf{p} - \frac{\mu}{r}$$ The angular momentum vector is given by: $\mathbf{L} = \mathbf{r} ...
2
votes
0answers
30 views

Should the answer to Einstein & Quantum gravity be found in conditions of our local observable universe? [duplicate]

It seems like the discourse over a unified Einstein and Quantum Gravity disregards the phenomena encountered in our local observable universe, such as we find throughout the Intergalactic Medium. One ...
0
votes
1answer
34 views

Right hand solenoid example

I am just studying for my physics final exam. I came across this example and am not satisfied with the answer. My answer is that Z is South and the current would flow from Y to X because of the ...
1
vote
2answers
66 views

Does a ball spin more from left to right than it does from right to left?

I swear that in a manual on tennis published by the German Tennis Association (Deutscher Tennis Bund) it stated as fact that all things being equal a ball will move more from left to right than from ...
1
vote
2answers
50 views

Determining Acceleration Based On Graph

I understand how to solve this problem, but I am unsure how to generate an equation for the graph (below). My current attempt involves using the mass provided along with the derivative of the line ...
0
votes
1answer
22 views

Dependence of scattering on particle size and refractive index

How does the intensity of light scattered by a medium depend on the refractive index and the size of the particles dispersed in the medium? I found plenty of literature about the dependence of ...
1
vote
0answers
23 views

Can I use ultrasound to detect metal underground? [on hold]

I am doing research which aim to detect metal underground. I know that most metal detectors are based on the concept of change of inductance of an inductor caused by mutual inductance with a ...
1
vote
2answers
52 views

On the pressure in a fluid

In my textbook (Resnick Halliday Krane), the derivation for pressure in a fluid at a given depth is done assuming that the fluid is homogeneous. Thus, the book concludes that the pressure in a fluid ...
1
vote
1answer
21 views

What is the magnetic field on surface of a current carrying conductor?

Biot Savart's law says there should be none as the current element is along the displacement vector, hence as $\sin \theta = 0$, so, the magnetic field must be zero. But, as per Ampere's loop rule ...
-2
votes
0answers
30 views

Finding the angle of fuzzy dice in a car when deccelerating [on hold]

You are driving at 25m/s when you see Bambi in the road! The coefficient of friction between your tires and the ground is .8. You slam on your brakes praying friction is great enough for Bambi to ...
3
votes
3answers
166 views

How is a photon measured?

If photons transmit the electromagnetic force, which is observable: the photon or the electron? Do we ever directly measure a photon, or do we only measure it's effect on electrons. For example ...
1
vote
1answer
26 views

Qualitative idea of viscosity [duplicate]

Suppose we are considering some fluid. As I currently understand, viscosity exists because of the following: supposing we draw one infinitesimal surface on the fluid, then the fluid parcels on either ...

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