Questions tagged [conservation-laws]
The statement that a property of a system does not change if the system is isolated.
2,670
questions
2
votes
0
answers
33
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?
0
votes
0
answers
41
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. ...
0
votes
1
answer
53
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?
2
votes
1
answer
50
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 ...
0
votes
0
answers
20
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 ...
2
votes
1
answer
53
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 ...
2
votes
3
answers
225
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 ...
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
votes
1
answer
34
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?...
0
votes
0
answers
37
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) ...
1
vote
0
answers
22
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:
$$\frac{\partial \rho}{\partial t}+\vec{\...
-2
votes
1
answer
51
views
If we know information existed when life first began on earth, then can’t we surmise that information existed prior to earth life? [closed]
If true, then wouldn’t information have been created when the universe was created?
In other words, if information existed from the start of the universe, then it’s possible that information can not ...
0
votes
1
answer
27
views
Noether current associated with transformation $\delta \psi=i\alpha \psi$
I'm doing problem 3 from sheet 2 of David Tong's lecture notes. We have given the complex field $\psi(x)$ which is governed by the Lagrangian
$$\mathcal{L}=\partial_\mu \psi^*\partial^\mu \psi -m^2\...
2
votes
5
answers
424
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
203
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 ...
0
votes
1
answer
48
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 ...
0
votes
1
answer
35
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 ...
0
votes
1
answer
7
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
vote
3
answers
104
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 ...
0
votes
3
answers
114
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 ...
0
votes
0
answers
64
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 ...
6
votes
6
answers
788
views
Energy to momentum
Is there anyway to convert energy to motion IN SPACE? Let's say a satellite collects electric energy from sun using solar panel. Is it possible to convert it to Linear motion? The only way I know to ...
19
votes
7
answers
2k
views
What makes energy "the" conserved quantity associated with temporal translation symmetry?
This kind of relates to my prior question about the non-triviality of temporal translation symmetry and will use some of the same concepts:
How is energy conservation & Noether's theorem a non-...
0
votes
0
answers
16
views
Equivalent wording of Tong's Noether's theorem statement for higher rank currents/charges
Tong states in equation 1.33:
$$
\frac{d Q_V}{dt} = - \int_A j\cdot dA
$$
"where A is the area bounding V and we have used Stokes' theorem. This equation means that any charge leaving V must be ...
6
votes
2
answers
1k
views
How can a pulsar slow down?
I saw in some astronomy textbooks that pulsars gradually slow down due to the loss of energy by its radiation. I wonder why this is possible?
Although the radiation is now not thermal but in the form ...
3
votes
1
answer
75
views
Why does angular velocity changes as i strech my hands?
Suppose I am sitting on a turnable chair. I have given that turnable chair some angular velocity $\omega$ and it starts rotating around me as axis of rotation.
Now at that instant suppose I expand out ...
0
votes
1
answer
24
views
When the mass flux is constant, why does $ v d\rho = - \rho dv$?
I'm reading about mass fluxes and I came across when the mass flux is constant $$ j = \rho v$$ then the following equation is true $$ v d\rho = - \rho dv$$ What does that last equation even represent? ...
0
votes
0
answers
21
views
Probability current density in a momentum eigen state seems counter-intuitive
The physical significance of probability current density (PCD) is usually given as a local conservation of probability. The time rate of reduction of probability at a point is accompanied by a local ...
2
votes
7
answers
308
views
Is Conservation of Linear Momentum subservient to conservation of Angular Momentum?
When particles physically interact, they transfer linear momentum and angular momentum between one another via force .
When particle P1 exerts force on particle P2, P2 exerts an equal and vectorially ...
-1
votes
2
answers
68
views
Why isn't momentum conserved here? [duplicate]
Suppose I throw a ball horizontally towards a wall with momentum $\vec p$. Let it collides with the wall and then rebound back towards me with momentum $-\vec p$. Since the wall remains stationary, ...
2
votes
2
answers
83
views
Is gauge symmetry necessary for charge conservation?
The common view is that gauge symmetry is necessary for conservation of charge(s) in Yang-Mills theory. But one thing I have never been able to get out of my head is, if there isn't any other possible ...
3
votes
1
answer
91
views
When we move do we "borrow" momentum from the earth?
In 2001 Arthur C Clarke wrote:
Like a ball on a cosmic pool table, Discovery had bounced off the moving gravitational field of Jupiter, and had gained momentum from the impact... Yet there was no ...
16
votes
11
answers
5k
views
Is angular momentum just a convenience?
I'm wondering whether angular momentum is just a convenience that I could hypothetically solve any mechanics problems without ever using the concept of angular momentum.
I came up with this question ...
1
vote
1
answer
27
views
Single gravitational plane wave or their interference can carry spin angular momentum?
I would be grateful if anybody could tell me if I had one gravitational wave in the form of a plane wave, it still would carry spin angular momentum? We know that gravitational waves are mostly the ...
0
votes
3
answers
67
views
What's wrong with this equation (Newton's Third Law & Energy Conservative Principle) [closed]
In an elastic collision, the energy is conserved. So
$$
KE_{1} = KE_{2} \\
\frac{1}{2}m_1\vec u_1^2 + \frac{1}{2}m_2\vec u_2^2 = \frac{1}{2}m_1\vec v_1^2 + \frac{1}{2}m_2\vec v_2^2 \\
-m_2 (\vec v_2^2 ...
1
vote
2
answers
87
views
Why can't we use conservation of angular momentum in this question? [closed]
A rod of negligible mass and length is pivoted at its centre. A particle of mass $m$ is fixed to its left end and another particle of mass $2 m$ is fixed to the right end. If the system is released ...
0
votes
1
answer
26
views
A question on angular momentum for a body with constant velocity
Let a particle be moving in space with constant velocity. We are required to show that for that particle , angular momentum is constant throughout the motion irrespective of origin we choose.
MY PROOF:...
2
votes
2
answers
54
views
Matter-antimatter and annihilation
In this question, it seems posed how a particle and its anti particle can get close to each to annihilate. One answer proposed that "The force involved in annihilation is normally either the ...
-1
votes
4
answers
105
views
Doesnt $E=mc^2$ contradict the preservation of charge?
If we say that any mass is the same as a given energy amount, so we in theory could turn any particle into energy, wouldn't that mean we could turn a proton or electron into energy without turning its ...
4
votes
0
answers
36
views
General Proof that Noether Charge Generates a Symmetry [duplicate]
I am trying to understand how one proves that a charge derived from the Noether procedure generates the corresponding symmetry. That is, I would like to prove that
$$[Q, \phi(y)] = i\delta\phi(y)$$
I ...
0
votes
1
answer
56
views
Is linear momentum of an open system conserved?
My understanding is that a system is a collection of particles.
And according to Wikipedia, a closed system is one that does not allow transfer of matter in and out the system. However, my textbook ...
1
vote
1
answer
45
views
"Contradiction" Between 2D Steady-State Continuity and Navier-Stokes Equations
I am looking for some clarification on the incompressible, 2D, steady-state, cartesian Navier-Stokes equations for flow through a straight cylinder (flow aligned with the $x$-axis, so $u_y =0$), with ...
4
votes
3
answers
869
views
How can satellites change direction without any medium in space? [duplicate]
How can satellites change direction without any medium in space? How do spaceships move in space if there is no medium?
How does Newton's third law of motion work in space?
2
votes
0
answers
40
views
Is this an allowed interaction?
I am looking to understand whether this reaction is allowed:
$$e^+ + e^- \rightarrow K^0 + \pi^0.$$
Baryon number is conserved
Charge is conserved
Total intrinsic spin of underlying particles is ...
0
votes
1
answer
36
views
Continuity equation in the impeller of a centrifugal pump
my question is about the consequence of the continuity equation in the impeller of a centrifugal pump. As the figure shows, the impeller spins with angular velocity $ \omega $. The fluid enters the ...
6
votes
2
answers
68
views
How is it possible to initialize a quantum state in a superposition of energy eigenstates?
There are many experiments that prepare quantum systems in superpositions of different energy levels. For example, it is common to prepare cavities in coherent states, which are superpositions of SHO ...
1
vote
2
answers
50
views
Can objects move during a collision-elastic collisions?
Can two objects move together during an elastic collision before moving apart? Or would this be considered an inelastic collision since during the time in which the objects were stuck moving together, ...
1
vote
4
answers
288
views
Conservation of momentum + conservation of energy + Newton's second law = Contradiction?
When a body with a mass of 1 kg moves at constant velocity of 1 m/s and collide (elastic collision) with a body with the same mass that's at rest, we know from conservation of momentum and ...
2
votes
1
answer
44
views
Angular velocity after collision of two curling stones
I am trying to create physics for game. I think I can figure out how the stones should move after collision...
however I am not sure how to figure out the angular velocity of stones after collision.
...
0
votes
2
answers
71
views
Where does the (-) sign come from in $\nabla \cdot \vec{J} + \frac{d \rho_v}{dt} = 0$?
Where does the (-) sign come from in $$\nabla \cdot \vec{J} = - \frac{d \rho_v}{dt}~?$$
Starting from $$I = \int_s \vec{J} \cdot d\vec{s} = \int_s \rho\vec{u} \cdot d\vec{s}$$
$$= \rho_v \int_s \vec{u}...