Questions tagged [conservation-laws]

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

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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?
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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. ...
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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?
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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 ...
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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 ...
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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 ...
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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 ...
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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) - \...
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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?...
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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) ...
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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{\...
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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 ...
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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\...
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5 answers
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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. ...
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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 ...
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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 ...
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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 ...
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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 ...
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(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 ...
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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 ...
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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 ...
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6 answers
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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 ...
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7 answers
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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-...
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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 ...
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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 ...
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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 ...
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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? ...
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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 ...
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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 ...
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2 answers
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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, ...
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2 answers
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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 ...
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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 ...
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11 answers
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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 ...
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1 answer
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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 ...
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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 ...
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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 ...
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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:...
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2 answers
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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 ...
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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 ...
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4 votes
0 answers
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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 ...
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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 ...
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"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 ...
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4 votes
3 answers
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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?
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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 ...
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1 answer
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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 ...
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6 votes
2 answers
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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 ...
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1 vote
2 answers
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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, ...
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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 ...
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2 votes
1 answer
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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. ...
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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}...
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