# Tag Info

Accepted

### A puzzle about relativistic spin

With all thanks to Valter Moretti, I believe we now have a complete solution to my original puzzle. In the identities for $\mathbf{K}$ and $\mathbf{J}$ that disagreed with the celebrated ones due to ...
• 193

### Is the kinetic energy of a rotating system just the sum of the instantaneous kinetic energies of the individual particles?

There's no "linear KE" nor "angular KE", there's only KE. KE of a particle. The KE of a particle is defined as $$K = \frac{1}{2} m |\mathbf{v}|^2 \ .$$ You just need to take the ...
• 10.7k

### Angular momentum quantum number $l$ either integer or half integer

The book is correct in using “either - or”. But the reason relies upon more fundamental instances of QM than the structure of angular momentum operator. More precisely a certain superselection rule. ...
• 74.7k
Accepted

### Doubt on conservation of angular momentum for Kepler's laws

The definition of a plane can be written as $\vec{a}\cdot \vec{r} = 0$, where $\vec{a}$ is any vector perpendicular to the plane. In this case, you have a vector quantity $\vec{L}$, which is from its ...
• 133k

### Doubt on conservation of angular momentum for Kepler's laws

The angular momentum ${\bf L}$ of a point-like particle is proportional to the vector product of position ${\bf r}$ and velocity ${\bf v}$. It is a vector orthogonal to the plane containing ${\bf r}$ ...

### Is it possible to reproduce tennis racket theorem instability with a gymbal under earth gravitational field?

I'm posting a new answer for the following reason: a totally different way of implementing a Dzhanibekov effect demonstration occurred to me. Manufacture an object that is spherical on the outside, ...
• 21.6k

### Is it possible to reproduce tennis racket theorem instability with a gymbal under earth gravitational field?

A gimbal suspension for the purpose of demonstrating the Dzhanibekov effect (when weightlessness is not available) presents special challenges. In a comment I already linked to the video about the ...
• 21.6k

### Tilting due to Earth's rotation at a distance

If you stand on a high tower, the velocity on the top is greater, than at the bottom, since the angular velocity is the same. So if you drop a stone it has compared to the bottom a relativ velocity ...
• 6,440
Accepted

### Angular momentum of matter halos?

The formula $L = mvR$ only applies to to a point mass $m$ rotating around some axis with radius $R$ with velocity $v$. In dark matter halos, you have a lot of particles moving in random directions, so ...
• 1,252

### How to understand gyroscopes by using vectors?

About the angular momentum vector: While efficient, the concept of angular momentum vector is itself already a mathematical abstraction. In order to grasp gyroscopic precession we need to see how it ...
• 21.6k
1 vote
Accepted

### Proving the Plane Harmonic Oscillator move in an ellipse

I have found this PDF (from which I'll just mostly copy the contents). It starts from the solution of the equations of motion, $$x=A\cos(\omega t+\alpha) \\ y=B\cos(\omega t + \beta).$$ It then says ...
• 56
1 vote

### In equation (20) from lecture 10 in Leonard Susskind’s ‘Classical Mechanics’, why is there a summation involved?

All the answers and comments above are brilliant and professional, however I probably get your point here, as I am also very new to these topics and notations. For the summation, it is not about any ...
1 vote

• 54.7k
1 vote

### Energy eigenvalue of hydrogen-like atoms using Laplace-Runge-Lenz vector

Your first task is to absorb all superfluous constants into your nondimensionalized variables, and do the same for the nice review by Valent which is required reading, if you cannot follow WP or Pauli....
• 64.5k
1 vote

### Reference on partial wave expansion in the context of QFT

Apart from Weinberg QFT vol1, section 3.7, one can look at the paper: "On the general theory of collisions for particles with spin", Jacob and Wick 1959. A recent review that I found useful ...
1 vote

### Why doesn't centrifugal force change instantly?

Looking from above, when any spinning wheel is nudged anticlockwise it always responds by trying to rotate the spin axis so that the axis is vertical and the wheel is rotating anticlockwise around the ...
• 6,464

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