# Tagged Questions

The statement that a property of a system does not change if the system is isolated.

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### Conservation of linear momentum and velocity of a system (damper and spring in a series)

This example is from a book on dynamics. Let us consider the system above formed by two blocks (each of mass $m$) connected by a linear damper and spring in a series. They slide without friction on a ...
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### Is there a way for an astronaut to rotate?

We know that if an imaginary astronaut is in the intergalactic (no external forces) and has an initial velocity zero, then he has is no way to change the position of his center of mass. The law of ...
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### Why is only angular momentum conserved for a planet and not linear momentum?

Suppose a planet is moving in an elliptical orbit with the Sun at one of its focii. I know that the forces in existence will be gravity which provides the necessary centripetal force. Now my book ...
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### How do higher-order optical chiralities look like?

The optical chirality of the electromagnetic field is a conserved quantity, analogous to the energy density, linear momentum density, and angular momentum density, which describes how chiral the EM ...
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### At what speed is the conservation of angular momentum carried out?

If the sun suddenly slowed down would pluto's orbit immediately speed up? If so, then isn't the information pertaining to the sun's angular momentum change being carried to Pluto faster than light? ...
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### General Definition of Steady State

According to many sources (including Wikipedia, Stephani&Kluge, D.J. Acheson) a steady state ist: In systems theory, a system in a steady state has numerous properties that are unchanging in ...
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### Angular momentum paradox with 2 identical gears

Consider two identical gears touching each other. The system is friction less. One has a handle that you use to apply a torque on the entire system. If you turn the handle, there will be a non-zero ...
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What's a good book (or other resource) for an advanced undergraduate/early graduate student to learn about symmetry, conservation laws and Noether's theorems? Neuenschwander's book has a scary review ...
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### A mysterious conserved quantity for a central potential

In teaching a course in classical mechanics and I have come across (from my predecessor) a to me mysterious conserved quantity. We are considering a gravitational (or electric) potential with the ...
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### What is the different between a dark state and a ground state?

In a atomic quantum system, typically discussing in quantum optics, there is something called dark state. A dark state is a state of a quantum system that does not emit any photon. A ground state also ...
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### Number conservation of bosons and fermions

Why is the number of bosons not conserved while the number of fermions is conserved? Does it have something to do with the Pauli exclusion principle?
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### How is angular momentum conserved

from a classical perspective, what is it about angular momentum fundamentally that means it has to be conserved? Surely if I have a rod about a fixed axis and a moving particle hits the end it will ...
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### Can a neutron decay to the gravitons?

Is it possible that a bunch of neutrons totally decay to the graviton? In other words, does the baryon number conserve in the quantum gravity interactions?
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### First law of thermodynamics with additional term

I read in a paper that a "known expression for the heat received by a body" is $$dQ=dU+pdV-\mathbf{v}\cdot d\mathbf{P}$$ where $\mathbf{P}$ is the linear momentum of the body, $p$ is the pressure, $U$...
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### Is the reaction force for a stone hitting a wall infinite?

Let us assume a rigid stone which moves in empty space with a constant speed of $v$. (Or in the air with no friction and drag or you can imagine a free fall with friction). This stone hits a rigid ...
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### Explain this contradiction of violation of “ energy conservation ” using classical mechanics

Consider a rocket moves upward with some acceleration for a very small time say '$dt$' then the kinetic energy increases (for acceleration). as well as as the potential energy increases (due to height ...
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### Motion in Spaces [duplicate]

How does a shuttle move in the space as, the whole mechanism of a rocket is based on 3rd law of newton but, we can not apply 3rd law of newton in space because there is no reactive force in space?
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### Gravity between two unequal masses. Do both masses move?

I've been watching videos about gravity and I have a question My understanding is that mass have gravity and gravity is a force which attract other object with mass. For example, I jump up and the ...
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### Dirac equation in the algebra of physical space and conservation laws

I have the following question: I was thinking, is it possible to obtain the conservation laws for the Dirac equation in the algebra of physical space? If yes, how? Can anyone show me a book for these ...
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### Energy conservation in electrodynamic system

Consider two charged particles initially at rest in the configuration below. Let us assume the following: Starting at time $t=0$, we apply a constant force $f$ to the the bottom particle so that ...
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### When are energy, mechanical energy, momentum, and angular momentum conserved?

I am in AP Physics and my only real hangup is knowing when the said quantities are conserved. Please define what is the SYSTEM in your answer. I kind of have the basic idea. For example, if there ...
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### Noether's Theorem for Hamiltonians and Lagrangians

Looking around I see one version of Noether's Theorem that creates conserved quantities from symmetries that preserve the Lagrangian (e.g. http://math.ucr.edu/home/baez/noether.html), and another ...
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### Applying conservation of energy to a railgun problem?

When a projectile is lunched due to the Lorentz force($F_L$), how can I apply the conservation of energy that the electrical energy inputted to generate the Lorentz force & magnetic field equals ...
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### Heat produced in collision [closed]

A 5-gram marble is moving at 5 m/s [N] collides with a 2-gram marble moving at 3 m/s [N]. Final velocity of 2-gram marble: 7 m/s [N] How would I find the amount of heat produced in this collision? ...
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### Why does light travel in straight paths?

Why light travels in straight paths? What's the real cause that makes light photons to go in a straight line? and what are the factors that could change the path of light externally? (I'm excluding ...