Questions tagged [conservation-laws]
The statement that a property of a system does not change if the system is isolated.
2,700
questions
-1
votes
0
answers
14
views
Unphysical solution to radial flow in a compressible fluid for Gaussian initial condition
Here's the problem I want to solve: a density of some fluid is moving radially outwards at constant velocity starting from a Gaussian initial condition, so the flux will be $\vec{j} = |V|\hat{r}$ and ...
- 193
1
vote
1
answer
37
views
Can a (conservative) four-force be derived from a scalar potential?
In special relativity, the four-force is the derivative of the four-momentum with respect to proper time (a Lorentz scalar):
$$f^\mu = \frac{\text{d}P^\mu}{\text{d}\tau} = c \frac{\text{d}P^\mu}{\text{...
- 590
1
vote
2
answers
65
views
How is there energy and charge in the universe? [closed]
According law of conservation of charge "the total electric charge in the universe is constant and charge can neither be created nor be destroyed" so how did the existing charge came. I f it ...
- 19
0
votes
2
answers
50
views
How does conservation of energy differ from conservation of momentum in this problem?
Here’s a problem and how I used two approaches:
https://ibb.co/r47C0gG
In the problem, I’m assuming there is no friction, so all energy and momunetum is conserved
How come the KE method isn’t the same ...
- 13
0
votes
1
answer
45
views
Singularities/infinities of continuity equation in polar coordinate
I encountered a bit of a difficulty in solving the continuity equation for polar coordinates. For a "fluid" or density of particles moving radially outwards with constant velocity, its flux ...
- 193
0
votes
0
answers
25
views
Momentum non-conservation while on-shell condition is satisfied
There are two particles $\it{N}$ and $\pi$ with masses $m_N$ and $m_\pi$ associated with Hermitian scalar fields $\phi_N$ and $\phi_\pi$. The matrix element for the process $N\rightarrow N'\pi$ is
$$...
-1
votes
1
answer
44
views
The divergence of the stress-energy tensor vanishes; is this statement sufficient to derive the Einstein field equations?
Can one derive the Einstein field equations from this statement alone?
$$0 = T^{\mu\nu}{}_{;\nu} = \nabla_\nu T^{\mu\nu} = T^{\mu\nu}{}_{,\nu} + \Gamma^{\mu}{}_{\sigma\nu}T^{\sigma\nu} + \Gamma^{\nu}{...
- 1,440
0
votes
3
answers
42
views
Momentum and energy conservation and preconditions
I'm reading Susskind's Classical Mechanics: The Theroretical Minimum and I also like to restrict my question to classical mechanics:
In chapter 4, momentum conservation is shown for a set of particles....
- 1,421
0
votes
1
answer
50
views
How to find distance between colliding objects?
Consider an object A with mass m with velocity v collides with another resting object B with mass M. After colliding we know that after some time both the objects will gain same velocity. But a ...
0
votes
0
answers
22
views
Does conservation of charge has anything to do with phase?
I was watching youtube lecture link and professor says that "the independence of that phase leads to conservation of charge" in the following equation.
$$\vec{\nabla} \cdot \vec{J} + \frac{\...
- 15
0
votes
0
answers
43
views
Newton's 3rd law and rocket nozzles
For a given solid propellent charge the energy given off by the charge is equivalent in all cases. Yet by varying the throat of the nozzle, the rocket velocity will be different for each individual ...
- 11
1
vote
1
answer
38
views
Superposition principle in classical collision theory
While reviewing my notes for a test, I stumbled upon a statement which I could not justify. In a diagonal two dimensional collision between a particle and a wall (considering the wall's mass as being ...
- 174
0
votes
1
answer
43
views
Can we deduce the conservation of mass in non-relativist physics or is it just an experimental fact? [duplicate]
It is a well-known fact that mass by itself is not conserved (since, for example, a particle can annihilate with its antiparticle). However, in classical physics, and as long as there is no physical ...
- 590
0
votes
1
answer
102
views
The effect of changing $g$ for an elastic bouncing ball [closed]
Suppose there's a bouncing ball who's collisions with the ground are elastic (so the ball's energy is conserved). If we were to then slowly adjust the gravitational acceleration $ g $ on the planet ...
- 11
1
vote
1
answer
35
views
Newton's Third Law in General Relativity [duplicate]
In the Framework of Newton's laws of motion gravity is a force. Therefore when a small body falls or is deflected towards a large mass like the Earth due to the force of gravity, it is said that the ...
- 2,499
0
votes
0
answers
31
views
Opposite helicities of $e^-$ and $\bar{\nu}_e$ from angular momentum conservation in pion decay
Consider the decay $\pi^-\to e^-+\bar{\nu}_e$ in the rest frame of the pion so that $L_\pi=0$. Since the pion is a spin-$0$ particle, $S_\pi=0$. Therefore, the total initial angular momentum $J_i\...
- 9,415
1
vote
1
answer
28
views
About applying the angular momentum conservation to $\beta$ decay or similar decays
Consider the $\beta^-$ decay:
$$n^0\to p^++e^-+{\bar\nu}_e.$$ The spin angular momenta of all the particles are given by $$S_n=S_p=S_e=S_{\nu_e}=1/2.$$ Therefore, by the addition of the angular ...
- 9,415
3
votes
2
answers
64
views
Is Carter constant exclusive to general relativity?
In other words, is there constant of motion analogous to Carter constant in any other field aside from general relativity?
I think since Carter constant is derived from Kerr metric - a metric ...
- 209
1
vote
1
answer
62
views
Why is I = $\partial Q / \partial t$ and not $I=-\partial Q / \partial t$?
I was playing around the Maxwell equations and I came across this:
$$\nabla\cdot J =-\frac{\partial \rho}{\partial t}$$
$$\iiint_V{\nabla\cdot J \space \partial V} = \iint_A{J\cdot\partial A}$$
$$-\...
- 274
0
votes
1
answer
70
views
Why is the center of mass of this system moving?
Here in this question , considering the system to be the thread , ladder and man , we can say that as the gravitational force which is and external force equal to 2Mg is acting in the downward ...
user327809
7
votes
2
answers
648
views
Do quantum measurements violate conservation laws?
When we measure the spin angular momentum of a particle in an axis different to its current spin, we change the direction of its spin, which taken by itself would be a violation of the law of ...
- 924
3
votes
1
answer
104
views
Why do "good" quantum states remain stationary under perturbation?
I've been reading the degenerate perturbation theory section of Griffiths QM. He introduces the idea that, if we can find an operator $\hat A$ which commutes with $\hat H^0$ and $\hat H'$, then ...
- 419
0
votes
2
answers
44
views
Lt. Joe Kenda's expertise(?) in fundamental physics
I may have gotten confused due to advanced age, but I heard Joe Kenda on the TV show "Homicide Hunter" make a statement that left me befuddled and in doubt. Of course, the lieutenant has ...
- 3
1
vote
1
answer
120
views
Law of conservation of momentum and interference [closed]
There are two guns a and b that emit electrons simultaneously. Electrons meet at a point с ...
- 113
-1
votes
2
answers
45
views
Conservation theorem for cyclic coordinates in the Lagrangian
Suppose $q_1,q_2,...,q_j,..,q_n$ are the generalized coordinates of a system.
$q_j$ is not there in the Lagrangian (it is cyclic).
Then $\frac{\partial L}{\partial\dot q_j}=constant$
In Goldstein, it ...
- 249
1
vote
2
answers
31
views
Question related to Mechanics of rigid bodies
]3
Why the initial momentum at the highest point became zero. why it can't be sum of those two Vertical components.
- 15
0
votes
3
answers
38
views
When does the angle matters in Work laws?
i've got this problem :
A 30.0-kg crate is initially moving with a velocity that has magnitude 3.90 m/s in a direction 37.0⁰ west of north. How much work must be done on the crate to change its ...
- 1
1
vote
0
answers
29
views
Angular momentum and energy conservation for a given Hamiltonian
Assume that the evolution of a system is defined by a Hamiltonian $H$ given by
$$
H= a \,\mathbf{p} \cdot \mathbf{p} + b \,\mathbf{p} \cdot \mathbf{q} + c \,\mathbf{q} \cdot \mathbf{q}.
$$
Here $a,...
0
votes
1
answer
38
views
Two objects of equal and opposite velocities collide elastically. If the two objects have different masses, which one has a bigger final speed? [closed]
Question: Two objects, one less massive than the other, collide elastically and bounce back after the collision. If the
two originally had velocities that were equal in size but opposite in direction, ...
- 103
2
votes
0
answers
35
views
Photon absorbed, identical Photon re-emitted [closed]
I believe I frequently see it stated that a photon is absorbed and an identical photon is emitted.
How can the energy in equal the energy out, with no loss?
- 77
0
votes
0
answers
45
views
What is the mathematical derivation for no diffusion term in the mass continuity equation of the Navier-Stokes/Euler equations?
In this post the fact that the mass continuity equation in a mixture of gases has no diffusion term, i.e.,
$$\frac{\partial\rho}{\partial t}+\nabla\cdot(\rho\vec{v})=0$$
has been discussed. ...
- 699
0
votes
1
answer
57
views
What are the symmetries of standard model and its conservation laws? [closed]
Is there like a list of symmetries in the standard model with associated conservation laws?
- 11
2
votes
1
answer
54
views
Physics of tennis hit
If one takes notice the tennis players hit the ball on the right corner that way: Their last step before the hit is on the right foot, then they hit and then their left foot goes up in the air about ...
- 659
0
votes
0
answers
21
views
Cosmic strings and conservation of momentum
I've been told that cosmic strings useful don't give of gravity.
I've been told that if an objects passive and active gravitational masses differ then this will result in a violation of conservation ...
- 1,436
2
votes
1
answer
61
views
Continuity and Bernoulli's Equation in a vertical pipe with different cross sections [closed]
Consider the following situation where the amount of water that goes through a cross section $A_1$ per second is the same as it goes through $A_0$ (just continuity) and is always constant:
I setup ...
- 21
2
votes
3
answers
227
views
Null conserved angular momentum
If the angular momentum of a particle is conserved and it is also 0, then is it true that the particle moves along a line? If so, how can we derive the equation for the trajectory from both the above ...
- 159
0
votes
0
answers
18
views
Boussinesq Approximation Term in Momentum Equation
According to the Boussinesq equation: $\rho = \rho_o ( 1 - \beta_t ( t – t_o) - \beta_c ( c – c_o ) ) $
So, $\rho \cdot g$ should be equal to : $\rho g = g \rho_o ( 1 - \beta_t ( t – t_o) - \...
- 1
-1
votes
1
answer
37
views
Why does production of electron come with either electron neutrino or positron?
I read that when an electron is produced, it always comes either with an electron neutrino or with a positron. Why is that so?
Why doesn't an electron come instead with say a muon neutrino or antimuon?...
- 3,895
0
votes
0
answers
38
views
Help with understanding virtual displacement in Lagrangian
I know that these screen shots are not nice but I have a simple question buried in a lot of information
My question
Why can't we just repeat what they did with equation (7.132) to equation (7.140) ...
- 227
1
vote
0
answers
23
views
Does the continuity equation for fluids simply state that the material derivative of density is 0?
The definition of material derivative is:
$$\frac{Df}{Dt}:= \frac{\partial f}{\partial t}+(\vec{v} \ \cdot \vec{\nabla})f $$
And the continuity equation is:
$$\frac{\partial \rho}{\partial t}+\vec{\...
- 227
-2
votes
1
answer
53
views
If we know information existed when life first began on earth, then can’t we surmise that information existed prior to earth life? [closed]
If true, then wouldn’t information have been created when the universe was created?
In other words, if information existed from the start of the universe, then it’s possible that information can not ...
- 3
0
votes
1
answer
30
views
Noether current associated with transformation $\delta \psi=i\alpha \psi$
I'm doing problem 3 from sheet 2 of David Tong's lecture notes. We have given the complex field $\psi(x)$ which is governed by the Lagrangian
$$\mathcal{L}=\partial_\mu \psi^*\partial^\mu \psi -m^2\...
- 11k
2
votes
5
answers
426
views
Is it possible to move without throwing or pushing another object or energy?
All kinds of movement occur when a thing throws something out or pushes something back and then the thing moves.
Like the car pushes the road back, the rockets throw gases at high speed to move. ...
0
votes
2
answers
207
views
Do photosynthesis and respiration violate the law of conservation of energy?
I don't know, if it's a physics question, biology or chemistry question but anyways here it is:
I have been taught that for making one molecule of glucose in photosynthesis 18 ATP molecules are used ...
- 451
0
votes
1
answer
49
views
Integrate continuity equation in QM
From Shankar's QM book pg. 166:
The continuity equation for probability density in QM is
$$\frac{\partial P(\vec{r},t)}{\partial t}=-\nabla \cdot \vec{j}(\vec{r},t),$$
where $P=\psi^*\psi$ is the ...
- 3,895
0
votes
1
answer
39
views
Can Energy and Momentum Conservation prevent Particle Interactions?
I understand that Quantum Numbers must be preserved during particle interactions, which prevents certain interactions from occurring.
However, as Energy and Momentum must also be conserved, are there ...
- 329
0
votes
1
answer
8
views
Spin conservation in indirect optical transitions in bilayer TMDC
While I was reading this paper (https://arxiv.org/abs/2108.09129), I got confused with the spin conservation in optical transitions in bilayer 2D semiconductors. In figure 3(d), indirect transition ...
- 1
1
vote
3
answers
109
views
(Why) Is orbital angular momentum conserved for point masses?
The introduction of the angular momentum as $\vec l = \vec r \times \vec p$ is also true for point particles. So $\vec l$ must refer to the orbital angular momentum (and not the "spin") in ...
- 398
0
votes
3
answers
116
views
Can conservation of angular momentum be proven?
It's been a while I was thinking about conservation of angular momentum. The fact which makes me uncomfortable is why does uniform angular velocity implies,
$$\vec{\tau}^{\text{EXT}}=0.$$
I was trying ...
- 410
0
votes
0
answers
70
views
Noether current for self-dual Yang-Mills theory
The Lagrangian for self-dual Yang-Mills theory, in spinorial notations is given by
$$\mathcal{L}= B^{a\, AB} (\partial_{A}{}^{A'} A^a_{A'B} + f^{abc} A^b_{A}{}^{A'} A^c_{A'B})$$
where $B^{a\,AB}$ is a ...
