Questions tagged [conservation-laws]

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

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How is there energy 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 ...
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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 ...
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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 ...
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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 $$...
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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}{...
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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....
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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 ...
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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{\...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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\...
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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 ...
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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 ...
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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: $$\nabla\cdot J =-\frac{\partial \rho}{\partial t}$$ $$\iiint_V{\nabla\cdot J \space \partial V} = \iint_A{J\cdot\partial A}$$ $$-\...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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.
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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 ...
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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,...
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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, ...
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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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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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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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