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How to model energy loss in a rotating body?

I recently asked a question about modeling instability in a rotating rigid body. I now realize that I was mentally confounding two different effects: The "Dzhanibekov effect" in which a rigid object ...
Joe Strout's user avatar
4 votes
1 answer
2k views

Gyroscope's motion explained by internal forces

I'm having a hard time trying to figure out the inner mechanisms behind a gyroscope's precession and nutation. To be very specific, I want to consider gyroscopes like this one: (source: wordpress.com)...
andrehgomes's user avatar
4 votes
3 answers
636 views

Why general relativity over other similar theories?

Since all theories where gravity is seen as a result of space curvature automatically satisfies the equivalence principle, why should one prefer GR over all other alternatives? Surely you can make the ...
Madde Anerson's user avatar
4 votes
1 answer
617 views

Why do we assume simply connected domains and continuously differentiable fields in electromagnetism theory?

In many textbooks, including Griffiths', they erroneously claim that a field is irrotational if and only if it is conservative (there exists a scalar potential). This is true only if the domain of ...
Mattia F.'s user avatar
  • 235
4 votes
2 answers
729 views

If a photon has no mass, how can it be attracted by the Sun?

I read that the photon doesn't have mass, but my teacher says that the photon has mass because the sun can attract it (like in the experiments to prove the theory of relativity). I think that there ...
dilver's user avatar
  • 51
4 votes
4 answers
364 views

Why is the speed of light used to define the fourth axis of spacetime?

The four axes of spacetime are $x, y, z$ and $ct$, where $c$ is the speed of light, and $t$ is time. Why is the speed of light (not any other speed) used to define the fourth axis of spacetime? If ...
Shen's user avatar
  • 1,633
4 votes
1 answer
4k views

Why does Hauksbee's electrostatic machine produce light?

I'm reading on the history of the discovery of electricity and the electron, and I've went from reading about Rutherford's gold leaf experiment all the way back to Francis Hauksbee's spinning glass ...
NLev's user avatar
  • 41
4 votes
8 answers
44k views

Do particles always flow from high to low pressure?

In a recent question, it was stated that particles in high pressure air always flow to lower pressure. In a pipe with a constriction, fluid flows from from low to high pressure after the ...
TiredRyan's user avatar
4 votes
3 answers
588 views

How does energy transfer between B and E in an EM standing wave?

I'm trying to understand how an electric field induces a magnetic field and vice versa, its associated energy, as well as relating it to my understanding of waves on a string. Using a standing wave ...
David's user avatar
  • 2,657
3 votes
1 answer
1k views

How to interpret phase diagrams?

I find quite difficult to interpret phase diagrams in general, for example I see people discuss them along the following lines: Here we see the coexistence line between liquid-solid phases.. a ...
user929304's user avatar
  • 4,625
3 votes
0 answers
795 views

Gauge freedom in Lagrangian corresponds to canonical transformation of Hamiltonian

I want to show that the gauge transformation $$L(q,\dot{q},t)\mapsto L^\prime(q,\dot{q},t):=L(q,\dot{q},t)+\frac{d}{dt}f(q, t)$$ corresponds to a canonical transformation of the Hamiltonian $H(p, q, ...
Thomas Wening's user avatar
3 votes
2 answers
399 views

If electrons are waves, how do they repel each other? [closed]

Louis de Broglie said that electrons are waves. But how they repel each other?
Poin's user avatar
  • 159
3 votes
0 answers
311 views

Non-local gravitational energy tensor

The well-known derivation of the Landau-Lifshitz gravitational energy pseudotensor, relies on several requirements: 1) that it be constructed entirely from the metric tensor 2) that it be index ...
lurscher's user avatar
  • 14.4k
3 votes
1 answer
2k views

What is the difference between unit and dimensions?

If I say Height of a block = 2m, then would "Height" be called as a dimension
Sanchit Saxena's user avatar
3 votes
2 answers
2k views

What all has intrinsic spin?

What does and does not have intrinsic spin? Wikipedia Spin (Physics) https://en.wikipedia.org/wiki/Spin_(physics) says: “In quantum mechanics and particle physics, spin is an intrinsic form of ...
Jim Graber's user avatar
  • 5,699
3 votes
1 answer
907 views

Rayleigh-Jeans Law

My question is simple, why do we believe Rayleigh-Jeans law to be absurd? Is the Ultra-violet catastrophe incorrect or is it only because we can not create or know of a perfect emitter? I am a bit ...
Alvin B. Tigner Jr's user avatar
3 votes
1 answer
180 views

Why is the central charge $c=9$ supersymmetry in the internal manifold?

In [2] (abstract [here]) (https://inspirehep.net/record/245643?ln=en) they say that, when compactifying any superstring theory, the six dimensional internal manifold must have $N=2$ supersymmetry with ...
soap's user avatar
  • 787
3 votes
2 answers
448 views

Is the triple point of water exactly 0.01 $\deg$C?

Is the triple point of water exactly $0.01 \deg$C, or is this an empirical value/is there some uncertainty to it? If so, how do we know it is exact, and why?
user107224's user avatar
3 votes
2 answers
1k views

What is Timelike Quantum Entanglement?

I came across a New Study at http://arxiv.org/abs/1101.2565. Which talks about Timelike quantum entanglement. What does that mean? Comment added by L.Motl: The same preprint has been discussed in ...
Kiran Kumar's user avatar
3 votes
2 answers
860 views

Distinction between state space and space of functions

In Quantum Mechanics a particle is described by its wave function $\Psi : \mathbb{R}^3\times \mathbb{R}\to \mathbb{C}$. In that sense, the state of a particle at time $t_0$ is characterized by a ...
Gold's user avatar
  • 35.8k
3 votes
2 answers
428 views

Determination of the ground state of a field theory

Consider the Spontaneous symmetry breaking in the theory $$\mathcal{L}=\frac{1}{2}\partial_\mu\phi\partial^\mu\phi-\frac{\mu^2}{2}\phi^2+\frac{\lambda}{4!}\phi^4.$$ By the ground state of a classical ...
SRS's user avatar
  • 26.3k
3 votes
4 answers
26k views

Is heat always associated with Light?

I have found that light always produces heat. The only cases I think heat is absent with light is Fluorescence and Phosphorescence (maybe because they emit low energy but maybe the heat is still ...
Kai's user avatar
  • 141
3 votes
2 answers
2k views

Factor of 4 discrepancy between integral of Planck's law vs Stefan-Boltzmann law

Consider an absolutely black sphere with unit surface area and temperature $T$. Planck's law states for the power emitted per unit area of the emitter per unit solid angle of emission per unit ...
Ruslan's user avatar
  • 28.8k
3 votes
1 answer
2k views

What is the advantage of using exponential function over trigonometric function in analyzing waves?

A.P.French in his book Vibrations and Waves writes: . . . Why should the exponential function be such an important contribution to the analysis of vibrations? The prime reason is the special ...
user avatar
3 votes
2 answers
873 views

Charging by induction

When we charge an conductor by induction and grounding, we first bring a negative charge to the conductor. As a result the mobile electrons of the conductor get repelled and stay far from the negative ...
ffahim's user avatar
  • 333
3 votes
2 answers
549 views

The Michelson-Morley experiment

A fundamental assumption in special relativity is that the speed of light is constant in all inertial reference frames, which was first established empirically via the Michelson-Morley experiment. ...
dezign's user avatar
  • 304
3 votes
3 answers
1k views

General Relativity 2-Body Closed Form

Is there a closed form solution in general relativity to the 2-body orbit problem?
James Bowery's user avatar
  • 1,327
3 votes
4 answers
4k views

Stress in a rod clamped between two rigid walls when the temperature is increased

The usual approach to calculate stress is to equate thermal expansion in the unclamped condition to the magnitude of contraction caused by strain produced due to the walls. I have some questions about ...
Aditya Prakash's user avatar
3 votes
3 answers
5k views

How can we see the moon while it's between the Earth and the Sun? [duplicate]

I know this sounds like (and probably is) a stupid question, but I can't figure it out. As far as I know, the crescent shape of the moon is when the moon is on the sunny side of the Earth, but that ...
imyxh's user avatar
  • 141
3 votes
4 answers
548 views

Entropy production in isolated system

My question relates to the stereotypical example for understanding the first and second laws: an isolated system filled with a gas of non-interacting molecules with constant ($E$,$V$,$N$) has two ...
Javi's user avatar
  • 961
3 votes
4 answers
2k views

How to explain what an electron is to someone new to physics? [closed]

I've got asked by someone who just graduated school and is about to start studying physics, what exactly is an electron, if it is not "a small ball rotating around the core of an atom". I couldn't ...
user avatar
3 votes
3 answers
1k views

Why are there $1 / 1.602176634 \times 10^{-19}$ electrons in a coulomb?

Why that exact number of electrons in one coulomb? who decided it? there is nothing wrong with the number, it just seems slightly messy. Why didn't the scientific community just settle on an easier ...
ipsa scientia potestas's user avatar
3 votes
1 answer
461 views

Angular equation for Schrödinger's equation in 3D

In Griffith's book, section 4.1.2, the angular equation for Schrödinger equation in 3D is expressed as $$\sin\theta \frac{\partial}{\partial \theta}(\sin \theta \frac{\partial Y}{\partial\theta})+\...
TaeNyFan's user avatar
  • 4,215
3 votes
2 answers
43k views

Difference between Speed and Velocity

What is the difference between Speed, Velocity and Acceleration? Could any one describe it pictorially?. I am more over confused even after investigating many times. I am unable to relate myself ...
ddpd's user avatar
  • 133
3 votes
2 answers
332 views

Are gravitational waves effected by the curvature of space time (gravitational lensing)?

I have a basic question I can't seem to find anything on (I keep hearing about how gravitational waves and gravitational lensing were both predicted by Einstein). We all know about the gravitational ...
HopelessN00b's user avatar
3 votes
1 answer
4k views

Why isn't a metal pot a faraday cage?

Someone left their cell phone here, it was ringing like crazy. I stuck it in a metal pot with a metal lid to shut it up, it still rang. I later put it in a safe, it still rang, but so muffled as to ...
Loren Pechtel's user avatar
3 votes
2 answers
1k views

What defines a cold plasma?

Been looking for this very simple answer for a while now, and google returns a face cream with the words cold plasma in it. Very frustrated. Just wondering, what constitutes a cold plasma in the ...
GCien's user avatar
  • 191
3 votes
1 answer
2k views

Why is the potential energy of a particle in a travelling wave maximum at the mean position?

I mean, we calculate the speed of a wave (in a string) by considering the tensional force as a centripetal one (that's obviously an approximation), so shouldn't potential energy decrease in the ...
Aabesh Ghosh's user avatar
3 votes
3 answers
589 views

Showing that light bends twice as much as newtonian gravity predicts with special relativity

Light bends twice as much due to gravity as Newtonian theory predicts as discussed in this related question. The argument couched in general relativity, quote from here. Since most objects move ...
Kevin Kostlan's user avatar
3 votes
1 answer
1k views

Physical understanding of Anderson (disorder) localisation

My current understanding is that waves in disordered potentials experience localisation due to interference effects. (eg an electron in a disordered medium tries to take different paths of effectively ...
user12800's user avatar
  • 113
3 votes
2 answers
310 views

Violation of Stefan's law when shining a light on a black body

Suppose there is a black body in a dark room and the room temperature is constant. Now a ray light is shone upon it. Since a black body absorbs all radiation that falls upon it, it must absorb more ...
Vaishakh Sreekanth Menon's user avatar
3 votes
3 answers
2k views

What is the distance between two objects in space as a function of time, considering only the force of gravity? [duplicate]

What is the distance between two objects in space as a function of time, considering only the force of gravity? To be specific, there are no other objects to be considered and the objects in question ...
A. Miller's user avatar
3 votes
4 answers
8k views

Why is the net work done in a pulley-string system zero?

In any pulley system, where the pulleys and strings are massless and frictionless, why is the net work done by Tension zero?
Shodai's user avatar
  • 683
3 votes
3 answers
6k views

Is the yellow we perceive when our eyes are hit by red and green light at the same time the same yellow that is at the yellow frequency/wavelength?

I am trying to understand how color works, and I am curious whether the yellow we perceive when our eyes are hit by red and green light at the same time is the same yellow that is at the exact yellow ...
Dmytro's user avatar
  • 175
3 votes
1 answer
597 views

A basic question about how to apply the gauge covariant derivative in Yang-Mills theory

I am sorry if this question is too stupid... We know that Yang-Mills equation (without source) can be written as $$D^\mu F_{\mu\nu}=0,\tag{1}$$ where $$D^{\mu}=\partial^\mu-ig A^{\mu}$$ and $$A^\mu=A^...
Wein Eld's user avatar
  • 3,621
3 votes
4 answers
817 views

Why is a temperature gradient set up in a heated rod? [duplicate]

Suppose a cylindrical rod is maintained at 100 degree Celsius and the other at 0 degree Celsius. My book says that after reaching "Steady State" the rod will have developed a constant temperature ...
Varun's user avatar
  • 41
3 votes
3 answers
16k views

Can Quantum Entanglement and Quantum Superposition be considered the same phenomenon?

Quantum entanglement is known to be the exchange of quantum information between two particles at a distance, while quantum superposition is known to be the uncertainty of a particle (or particles) ...
Zero's user avatar
  • 109
3 votes
1 answer
322 views

If Fine Structure Constant in not uniform in space what would that imply for cosmic background?

Recently J.Webb submitted paper which has extraordinary claim - that Fine Structure Constant is different in different directions in space! He (with others) measured $\alpha$ using quasar spectrum ...
Karolis's user avatar
  • 483
3 votes
1 answer
2k views

Clarification in deriving the radial momentum operator $p_r$

In deriving an expression for $p_r$, a particle's radial momentum, I am unsure what is happening at a certain step. The derivation given in The Physics of Quantum Mechanics by Binney and Skinner is as ...
Nick Chapman's user avatar
3 votes
2 answers
3k views

Electron Double Slit Experiment-de broglie wavelength relation to distance btw slits

In the 2 slit experiment with electrons, is the distance between the slits related to the individual electron's de broglie wavelength? In other words, if the slits are too far apart which would ...
Greg 's user avatar
  • 53

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