All Questions

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0answers
5 views

The spring paradox: Is there any rational transformation for the relativistic force?

A simple thought experiment crossed my mind a few years ago for which I could not as yet find a convincing answer: Assume that a mass-less spring $S_1$ with (proper) constant $k_0$ hangs from the ...
0
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0answers
6 views

A bar weighs 2N and is 5m in length. What is the weight(M) needed to balance the bar?

A bar weighs 2N and is 5m in length. What is the weight(M) needed to balance the bar? I think I understand that the equation not including the weight of the bar would have 3*3=(5*1)+(2*X) M=4. I don'...
0
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0answers
5 views

Energy of a three-dimensional system

The energy of a three-dimensional system of dimensions Lx, Ly, Lz, is given by, $\varepsilon\left(n_{x}, n_{y}, n_{z}\right)=\frac{\hbar}{2 m}\left[\left(\frac{\pi n_{x}}{L_{x}}\right)^{2}+\left(\frac{...
0
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1answer
7 views

Why do we use a cylinder as a Gaussian surface for infinitely long charged wire?

Why do we use a cylinder as a Gaussian surface for infinitely long charged wire and not some other shape like cube?
0
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0answers
10 views

What is this text talking about (about change of spacetime co-ordinate systems)?

I'm completely lost on the last paragraph. My confusion is out of this world. Can someone explain this text part by part?
0
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0answers
12 views

How can beta plus decay be possible?

$\beta +$ decay is where a proton gets turned into a neutron and a positron and a neutrino. However, a neutron is heavier than a proton, so obviously this reaction is endothermic. So then, why does ...
0
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0answers
8 views

Electric current dipole moment

Electrical Current Dipole of dipole moment $\mathbf{p_{EC}}$ has unit of $A\cdot m$. My knowledge of dipole moments unfortunately stopped in high school where I learned that dipole moment $\mathbf{p}$ ...
0
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1answer
13 views

Fermi level in metals and semicoductors

As we read about Fermi energy, it is the energy of the highest occupied electrons. But if we look at the energy level diagram of semiconductor the Fermi level is situated somewhere between the valence ...
0
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0answers
6 views

Conductance Charged Box

A box with thick and conductive sides with dimensions: Lx, Ly, Lz- while LZ>>Lx,Ly. The long dimension of the box [Lz] can be viewed as infinite so Phi(x,y,z)=Phi(x,y) Given that: Phi(0,y)=-Phi(Lx,...
0
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0answers
9 views

Radius vs Speed for Flywheel energy storage

If I understand the foemula correctly the equation for kinetic energy of a flywheel is mw^2r^2 whereas the formula for "centrifugal force" is mw^2r. So how come so much focus is on the speed of ...
0
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1answer
21 views

Writing a two variable function$ f(x,t)$ in terms of Dirac-Delta $δ(x)$ function and a function $P(t)$?

How to write a two variable function $f(x,t)$ in terms of Dirac-Delta $\delta(x)$ function and a function $P(t)$? For example; I read something in a book. You can find the following picture. But ...
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0answers
8 views

Physics-y resource request on Killing-Cartan forms

Most books that treat nonabelian gauge theory do not contain detailed discussion on Killing-Cartan forms, they'll usually just say that in $\text{SU}(N)$ Yang-Mills theory, one can choose generators $...
0
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0answers
11 views

Acoustics in heterogeneous medium: Reflection and Tranmission

I am trying to understand the effect of changing impedance on acoustic pressure wave. I have attached the image from the book "Fundamentals of Acoustics"(Kinsler et. al)(r1 and r2 are the impedance of ...
1
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1answer
15 views

Boundary condition for Hamilton-Jacobi PDE

For separable solutions to Hamilton-Jacobi PDE (say in 2D), we treat the Hamilton's principal function $S$ as $$S= W(x) + W(y) - E*t$$ and treat the separate parts as constants and find $W(x)$, $W(y)$...
0
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3answers
41 views

How do I understand Kinetic energy formula? [duplicate]

$$\frac{mv^2}{2}= Kinetic Energy$$ Can you explain me? What is purpose of $v^2$, $mv^2$, I am trying to understand the formula.
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0answers
9 views

Acoustic Plane Wave

Consider an ideal fluid which we have a spherical object on it and a progressive plane wave is heading the object. I have a simple question and it is: If we assume the amplitude of the incident ...
1
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0answers
16 views

Adding new fermions to a two-dimensional space

Suppose we have a two-dimensional space where we can seat electrons. Now, is it so that even before we start adding electrons the single electron energy levels already exist and as soon as we begin ...
0
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1answer
26 views

Is there differential form notation for Maxwell's equation in curved spacetime?

In special relativity, Maxwell's equations may be written as \begin{align*} dF = 0, \\ \star\, d\star F = J. \end{align*} In four-vector notation, this translates to $\partial_{\mu}F^{\mu\nu} = J^{\nu}...
-2
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0answers
33 views

My new equation of motion is right or wrong [on hold]

Every equation includes acceleration, but form the three equations I made forth equation of motion.![enter image description here](https://i.stack.imgur.com/IUrcH.
0
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2answers
22 views

Why there are three reactions at fixed beam in F.B.D?

It is fixed so no reactions will be present, then how three reactions at fixed beam in F.B.D? In F.B.D, we draw two reactions and moment reaction at fixed beam: Why there are three reactions at ...
2
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2answers
59 views

Why is effective mass a tensor?

So I came across the effective mass concept for solids the other day. It was mentioned that the effective mass is a tensor and may have different values in different directions. However, this is stark ...
0
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1answer
20 views

How will a polar molecule affect another molecule nearby, why it feels a relatively large force?

Mean position of the negative charges and positive changes are not at the same point (polar molecule) how will that affect another molecule nearby, why it feels a relatively large force? Will the type ...
0
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1answer
26 views

Is friction a conservative force when a ball rolls down an inclined plane(pure rolling)?

My book says friction is non dissipative for a cylinder rolling down an inclined plane,so is it conserved?
1
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1answer
37 views

Nuclear Physics Question

How could I calculate the chance of a proton actually joining the nucleus and 2 protons and 3 and so on? Assuming a large number of protons are fired at a substance?
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0answers
23 views

How small does a hole in a cup have to be to hold water without dripping? [duplicate]

I am creating a drain that lets water in but keeps pieces of plastic out. I need to know the size of a standard water droplet.
2
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1answer
39 views

In Feynman lecture Vol1. chapter12 about friction, $F = μN$ will fail because of the excessive heat generated. Why is that?

$F = μN$ will fail because of the excessive heat generated if the normal force or the speed of motion gets too big. Why is that? "$F = μN$, where μ is called the coefficient of friction (Fig. 12-1)....
0
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1answer
38 views

Oscillating spring, speed close to the equilibrium: How is this answer not 1.5? [on hold]

I have this question with the answer listed as $2.0\,\mathrm{m/s}$. "A $1.25\,\mathrm{kg}$ mass on a spring with a constant of $12.0\,\mathrm{N/m}$ is oscillating back and forth. Its maximum ...
1
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1answer
21 views

Klein-Gordon equation propagators: intersection with the support of the source

Let $(M,g)$ be a globally hyperbolic. Let $P = \Box - m^2$ be the Klein-Gordon differential operator. Following Fewster's notes, we may define the retarded/advanced propagators $$E^\pm : C^\infty_0(M)\...
0
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1answer
33 views

Muzzle Velocity of a Bullet out of a Barrel

We want to know the speed of a bullet, in feet per second, for a given round. We know the weight of the bullet, volume of the casing, amount of gun powder, length of the barrel, and diameter of the ...
1
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1answer
39 views

Tadpole diagrams in 1-loop massive scalar amplitudes?

Consider a massive scalar diagram such as or The loop momentum enters and exits the tadpole vertex, so that in the first diagram the momentum in the propagator connecting the two vertices is zero ...
0
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1answer
18 views

Does magnetic field make particle move in the direction of magnetic field(In circles)?

So my task says: particle with mass m moves freely through space and there is no electromagnetic field. In moment t = 0 uniform magnetic field is turned on with the field strength = B, and direction ...
0
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0answers
6 views

In Feynman lecture vol1,chapter12 friction between two wet glasses, is there any atmospheric pressure related during the process?

In Feynman lecture vol1,chapter12 friction between two wet glasses, is there any atmospheric pressure related during the process? Does the water between two glasses make use of atmosphere pressure? Or ...
1
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0answers
32 views

How far can a crow fly?

This question is all about order of magnitude estimation. All of my undergraduate physics classmates (and myself of course) were presented this question one afternoon (my god, that was over 50 years ...
1
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1answer
31 views

Entanglement of Werner States

Let the Werner state $$\rho_W = W\mid\Psi^-\rangle\langle\Psi^-\mid + \frac{1-W}{4}\mathbb{I},\ W\in [0,1].$$ I have repeatedly heard that such a state is separable if $W\leq\frac{1}{3}$ and entangled ...
1
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1answer
19 views

Derivation of Raychaudhuri equation - Trace

In Wald (Wald: General Relativity on page 218, equation 9.2.10) is stated that $$v^c∇_cB_{ab}=−B^c_bB_{ac}+R^d_{cba}v^cv_d $$ and to continue in order for the equation to be derived one needs to take ...
0
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2answers
25 views

Radio wavelength to penetrate 8 ft of seawater

I am trying to penetrate about 8 ft of seawater with a radio wave. What wavelengths will work? I understand that low wavelengths penetrate seawater the best, but I am looking for a frequency that does ...
0
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1answer
28 views

Pendulum attached to Oscillating Fulcrum

In the specific scheme, I would expect that by pulling the spring to the right, the pendulum due to inertia would have to move to the left quartile. In order to find the r vector: r = R + l and ...
0
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0answers
19 views

Resources on Physical Baryon States as mixings of multiplet states

I've been told, and have seen in papers (search for "mixing") that baryon resonances are not plucked from pure group theoretic multiplets, but are most often found to be mixings of states in different ...
-3
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3answers
91 views

Fire in my building [duplicate]

If there is a fire in a building and I am on 10th floor and I can't exit the building.Is it safe to jump out of the window with a chair and when chair is close to the ground I just jump of the chair ...
2
votes
2answers
65 views

Quantum corrections in path integral

I am working the following exercise: Calculate the generating functional $$Z[j]=\int \mathcal{D}\Phi \exp\left(\frac{i}{\hbar}S[\Phi,j]\right),\quad S[\Phi,j]=\int d^4x(\mathcal{L}(\Phi)+j\Phi),$$ $...
4
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4answers
572 views

Can two or more lightbeams (from a laser for example) have visible interference when they cross in mid-air?

I am wondering if two or more specific (in terms of frequency, wavelength etc) light sources could exist and capable of transmitting visible or invisible 'light', and where the beams intersect each ...
0
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0answers
12 views

Explicit form of Klein factors in Giamarchi

In Giamarchi, Quantum Physics in One Dimension, Appendix B, I don't understand how he did his last step in equation B.8, as shown below. If anyone has gone over the derivation, I would really ...
1
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0answers
17 views

Temperature anisotropy in plasmas

If a species of a plasma is described by a probability distribution function $f(\mathbf{r},\mathbf{v},t)$, is it possible to have different temperature (thermal energy) values for different directions ...
7
votes
1answer
480 views

Why don't all electrons contribute to total orbital angular momentum of an atom?

There are 47 electrons in a Silver atom, but talking about its orbital angular momentum we only take the outermost valence electron which occupies the 5s orbital. Why don't the remaining inner 46 ...
1
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0answers
28 views

Relativistic black and white image

A black and white image is represented by a scalar field $T = T(\vec{r})$ with $T(\vec{r}) = 0$ or $1$ indicating if the point $\vec{r}$ is white or black. Suppose the image moves with a relativistic ...
0
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2answers
45 views

During fusion (in the Sun), how is the repulsive effect of the strong nuclear force overcome?

During fusion (in the Sun or in attempts on Earth), how is the repulsive effect of the strong nuclear force overcome? (I assume from the name that fusion is just the combining of nuclei, but please do ...
1
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0answers
37 views

Wave equation calculation

when I have a 1 dimensional, linear wave equation: $\Box\ \psi(t,z)=(\partial_z^2-\frac{1}{v^2}\partial_t^2)\ \psi(t,z)=0$. $\Box$: d´Alembert operator So linear combinations of plane waves $e^{ik(...
0
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0answers
80 views

How do you derive a wave function?

I know this may be a dumb question to ask, but I'm a middle school student studying quantum physics out of interest and want to know how to find the wave function. Preferably in a square well.
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0answers
27 views

Unitary equivalence of two representations

Suppose we make a transformation from Minkowski space to another coordinate system. What does it mean to say that the two spaces are unitarily equivalent? I have often seen the comment that if the ...
1
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4answers
63 views

What makes air a good insulator?

What makes air a good insulator? My understanding of air is that its mostly just empty space with a few molecules whizzing around. Now consider a negatively charged ball suspended in air. Why don't ...

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