The conservation-laws tag has no wiki summary.
11
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4answers
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If all conserved quantities of a system are known, can they be explained by symmetries?
If a system has $N$ degrees of freedom (DOF) and therefore $N$ independent1 conserved quantities integrals of motion, can continuous symmetries with a total of $N$ parameters be found that deliver ...
12
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6answers
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How can there be net linear momentum in a static electromagnetic field (not propagating)?
I understand from basic conservation of energy and momentum considerations, it is clear in classical electrodynamics that the fields should be able to have energy and momentum. This leads to the usual ...
13
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6answers
2k views
Can Noether's theorem be understood intuitively?
Noether's theorem is one of those surprisingly clear results of mathematical calculations, for which I am inclined to think that some kind of intuitive understanding should or must be possible. ...
8
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4answers
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Deriving Newton's Third Law from homogeneity of Space
I am following the first volume of the course of theoretical physics by Landau. So, whatever I say below mainly talks regarding the first 2 chapters of Landau and the approach of deriving Newton's ...
18
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1answer
1k views
What conservation law corresponds to Lorentz boosts?
Noether's Theorem is used to related the invariance under certain continuous transformations to conserved currents. A common example is that translations in spacetime correspond to the conservation of ...
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0answers
558 views
Do symmetries increase the number of conserved quantities? [closed]
Let us consider a classical mechanical system of N particles in a constant external field. We have 3N coordinates and 3N velocities, so totally 6N unknown variables. We have 6N ordinary differential ...
12
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2answers
4k views
Why can't energy be created or destroyed?
My physics instructor told the class, when lecturing about energy, and that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
4
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1answer
284 views
Noether theorem and classical proof of electric charge conservation
How to prove conservation of electric charge using Noether's theorem according to classical (non-quantum) mechanics?
I know the proof based on using Klein–Gordon field, but that derivation use ...
12
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5answers
1k views
What is the conserved quantity of a scale-invariant universe?
Consider that we have a system described by a wavefunction psi(x). We then make an exact copy of the system, and anything associated with it, (including the inner cogs and gears of the elementary ...
4
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3answers
360 views
Noether theorem, gauge symmetry and conservation of charge
I'm trying to understand Noether's theorem, and it's application to gauge symmetry. Below what I've done so far.
First, the global gauge symmetry. I'm starting with the Lagragian
...
9
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1answer
237 views
Neutrino Oscillations and Conservation of Momentum
I would like to better understand how neutrino oscillations are consistent with conservation of momentum because I'm encountering some conceptual difficulties when thinking about it. I do have a ...
4
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4answers
505 views
What does it mean, when one says that system has N constants of motion?
For example for an isolated system the energy $E$ is conserved. But then any function of energy, (like $E^2,\sin E,\frac{ln|E|}{E^{42}}$ e.t.c.)
is conserved too. Therefore one can make up infinitely ...
11
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3answers
769 views
Decay of massless particles
We don't normally consider the possibility that massless particles could undergo radioactive decay. There are elementary arguments that make it sound implausible. (A bunch of the following is ...
-4
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1answer
182 views
Tricky Conservation of Momentum problem: find the ratio of the carts by mass percentage lost [closed]
A wagon is coasting at a speed $v_A$ along a straight and level road. When 42.5% of the wagon's mass is thrown off the wagon, parallel to the ground and in the forward direction, the wagon is brought ...
17
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5answers
812 views
Is the converse of Noether's first theorem true: Every conservation law has a symmetry?
Noether's (first) theorem states that any differentiable symmetry of the action of a physical system has a corresponding conservation law.
Is the converse true: Any conservation law of a physical ...
6
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3answers
419 views
How does $F = \frac{ \Delta (mv)}{ \Delta t}$ equal $( m \frac { \Delta v}{ \Delta t} ) + ( v \frac { \Delta m}{ \Delta t} )$?
That's how it's framed in my Physics school-book.
The question (or rather, the explanation) is that of the thrust of rockets and how the impulse is equal (with opposite signs) on the thrust-gases and ...
12
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3answers
703 views
No hair theorem for black holes and the baryon number
The no hair theorem says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge.
For me it is puzzling why ...
3
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4answers
867 views
Energy conservation in Electrodynamics
Let us suppose that we have a known electromagnetic wave-train of finite size propagating in a certain direction. There is a probe charge on its way. This EMW is an external field for the charge. The ...
1
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1answer
144 views
Improved energy-momentum tensor
While still dealing with this issue, I've stumbled upon this answer to a question asking about the conserved quantity corresponding to a scaling transformation. It mentions that in accordance with ...
5
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1answer
192 views
Effect of the tail of the cat in the falling cat problem
To explain why a falling cat can turn by 180 degree without external torque and without violation of the conservation of angular momentum, one usually models the cat as two cylinders as in
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2
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2answers
137 views
Does a constant factor matter in the definition of the Noether current?
This is a very basic Lagrangian Field Theory question, it is about a definition convention. It takes much more time to typeset it than answering, but here it is:
Consider a field Lagrangian with only ...
6
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1answer
159 views
Can a deformable object “swim” in curved space-time? [duplicate]
Possible Duplicate:
Swimming in Spacetime - apparent conserved quantity violation
It is well known that a deformable object can perform a finite rotation in space by performing deformations ...
3
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3answers
393 views
Newton's 3rd Law: How can I break things?
If I punch a wooden board hard enough and it breaks in two, has the board still exerted a force of equal magnitude on my fist?
When the board breaks in two due to my force, the halves have a ...
3
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1answer
177 views
Lepton Number Conservation
What is the global symmetry of the electroweak Lagrangian that gives rise to lepton number conservation?
As I understand it, electric charge is some linear combination of the conserved quantities ...
3
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3answers
297 views
Is there more energy in the collapse of a cavitation bubble than the energy required to create the bubble in the first place?
The following does not include all scientific details and parameters, only a common summary of "thoughts". What is scientifically wrong with this summary?
When you take your beer and tap the top ...
2
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3answers
251 views
Train crash: are these situations alike?
I was just wondering... I believe that if a car travelling 50 miles per hour crashes into a wall, the result should be the same as crashing to another car also travelling 50 miles per hour (but in the ...
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1answer
71 views
How to get the new direction of 2 disks colliding?
I'm developing a 2D game including collisions between many disks. I would like to know how I can get the angle corresponding to the new direction of each disk.
For every disk I have this information ...
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3answers
130 views
Displacement with zero velocity
I know that we can rotate a deformable object using internal forces only in space. Thus we can cause an angular displacement without the use of any external forces.
The following youtube video shows ...
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1answer
346 views
What causes a force field to be “nonconservative?”
A conservative force field is one in which all that matters is that a particle goes from point A to point B. The time (or otherwise) path involved makes no difference.
Most force fields in physics ...
0
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1answer
145 views
What happens if object is thrown in empty space?
If I throw a object in empty space, I apply a force to throw that.
Then it gains some acceleration and it's speed increases.
So will it's speed keep on increasing, or it will get stable?
If yes, ...
-1
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1answer
734 views
Violation of Newton's 3rd law and momentum conservation
Why and when does newtons 3rd law violate in relativistic mechanics? Check this link http://www.animations.physics.unsw.edu.au/jw/Newton.htm.
