# All Questions

140,122 questions
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### 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 ...
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### 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'...
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### 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 ...
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### 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 $W(x)$, $W(y)$...
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### 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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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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(...
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### 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.