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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 to figure out whether a force mediator is $W^+$ or $W^-$ in Feynman diagram?

How to figure out whether a force mediator is w+ or w- in feynman diagram? I always make sure each vertex is zero but sometimes I get my w+ or w- worng.
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2answers
68 views

Quantum Tunneling and Conservation of Energy

According to my understanding quantum mechanics, the probability of any particular particle in the universe being at any specific location in the universe is very small but never actually becomes zero....
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1answer
49 views

Conservation of lepton

I was reading a bit about nuclear/particle physics when I came up to Cours et problèmes corrigés de physique nucléaire et de physique des particules by Philippe Miné in 2016 ISBN 2340011566, which is ...
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4answers
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Can every mass obey the Newton's 3rd law?

As in, if we were able to produce a very huge amount of force on a very small body, would it push back with the same force? Given it doesn't break or disintegrate. Like if we electromagnetically put ...
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Why is the photon emitted from a bound electron dropping energy levels not exactly equal to the difference in energy levels [duplicate]

In a problem sheet there was a question asking us to guess why a photon released from an electron dropping energy levels does not have the exact difference. My guess is that the electron needs some ...
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1answer
57 views

Liouville's integrability theorem: action-angle variables

For classical dynamical systems, let $I_{\alpha}$ stand for independent constants of motion which commute with each other. Pg 323 of Jose-Saletan and also 'Remark 11.12' on pg 443 of Fasano-Marmi's '...
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Rotationally invariant metrics and conservation of angular momentum

This was prompted by an exam question, though the questions are more general: A 2D Riemannian space has the metric: $ds^2=dr^2 + \gamma^2 r^4 d\phi^2$ State what conserved quantity ...
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Information cannot be lost [closed]

What does information cannot be lost mean. Not why it can't be lost. Just wat does information cannot be lost itself. Okay I have it. If it were possible to lose information give me a specific piece ...
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4answers
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Why is momentum not conserved in this particle interaction?

In this particle equation. $$\Lambda^0 \rightarrow n$$ I thought it would be possible to conserve momentum as the neutron could carry on in a straight line and adjust its speed accordingly so the ...
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5answers
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Isn't Kirchhoff's junction law a violation of conservation of charge?

As a beginner of classical electrodynamics I am quite confused in understanding Kirchhoff's junction law or it may be the reason that I misunderstood the law of conservation of charge.Please correct ...
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3answers
129 views

Gauge Symmetry & Off-Shell Current Conservation in QED

I’m reading Srednicki - I’m quite confused on bottom page 351 to top page 352 (which I recap below): This is his discussion in a nutshell. The Dirac Field has a conserved current which follows from ...
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1answer
53 views

Will the mass of a stick of dynamite be conserved when it is exploded?

If a stick of dynamite is blown up in a hermetically sealed chamber, the mass of the chamber and fragments, the heat, sound, and light would still be equal to the original mass of the chamber and ...
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1answer
34 views

Angular momentum conservation during collision

If I have a disk which is pure rolling and it strikes with a ladder, so can I conserve angular momentum about point O? I think I can because normal reaction passes through O, so torque due to it will ...
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Why is the decay $\rho ^+ \rightarrow \rho ^0 \pi^+$ allowed by parity conservation and angular momentum conservation?

In the following decay: $$\rho ^+ \rightarrow \rho ^0 \pi^+$$ where $\rho^+$ and $\rho^0$has $J^P = 1^-$ and $\pi^+$ has $J^P = 0^-$ The parity conservation $P$ entails that $L$ (orbital angular ...
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0answers
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Interpreting the conserved charge in scalar QED

In scalar QED, applying Noether's theorem for internal global symmetries results in a Noether current that is dependent on the gauge because of the presence of the covariant derivative. $$j_\mu=-i(\...
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1answer
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What happens when in space a basketball flies and hits another resting basketball?

We have the following situation: In space, a basketball flies and hits another resting basketball. What happens next? a) Both balls fly in the original direction (but slightly slower) = momentum ...
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1answer
30 views

How to obtain the Noether charge for two interacting fields. Correct mode expansion for field operators

If I have two interacting fields $$ \mathcal{L} = \frac{1}{2}(\partial_\mu \phi_1)^2 - \frac{1}{2}m^2\phi_2^2 + \frac{1}{2}(\partial_\mu \phi_2)^2 - \frac{1}{2}m^2\phi_2^2 - g^2(\phi_1^2 + \phi_2^2)^...
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What is the physical meaning of a Supercharge? [duplicate]

What is actually being conserved? I've calculated it for the Wess-Zumino model but I still have no idea what is actually being conserved due to Noether's Theorem.
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0answers
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Does gravity follow law of conservation of energy? [duplicate]

Law of conservation of energy states that energy cant be created nor destroyed.I dont understand how it is applicable for a object moving under influence of gravity.If some force has been applied on ...
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48 views

Conservation of a topological current

I am trying to prove the conservation of a topological current, as you can see in the picture. I show that the two of the three terms vanish. However, the last one doesn't. Any suggestions/hints?
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Determining Whether a Given Hamiltonian is Conserved

in simple terms I'm looking to understand how we can tell whether a given Hamiltonian (or one that we've deduced) is conserved or not? I've tried looking at other similar questions but am not sure I ...
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2answers
810 views

Can muons decay into quarks?

Muon decays are almost always written as $$\mu^- \rightarrow e^-+ \bar\nu_e +\nu_\mu.$$ The reason given on wikipedia is that one of the product neutrinos of muon decay must be a muon-type neutrino ...
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1answer
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Collision angle relation to velocity of balls in Billiard Pool Game

I have a question about the billiard game physics. I know that in a billiard game, balls collisions are elastic and therefore the angle of the collision of two balls can easily be calculated no matter ...
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1answer
39 views

Killing tensor and Conserved quantities

The definition of the killing tensor is written above, as taken from Wikipedia. My question here is two-fold: Can all Killing tensors be build from the Killing vectors of that spacetime? Do Killing ...
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1answer
78 views

Why does leaking truck keep same speed? [closed]

Why this is the right answer? As we know liner momentum is conserved so: $$V_1 M_1=V_2M_2$$ and since truck is leaking oil $M_2$ is less than $M_1$, let $M_1=10$ and $M_2=8$ then $$V_1\cdot10=V_2\...
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Noether current and continuity equation in classical scalar QED

Consider the following scalar QED model \begin{align} S = \int \mathrm{d}^{d+1} x\, \left\{-\left(\mathrm{D}_{\mu}\phi\right)^{\dagger} \left(\mathrm{D}^{\mu}\phi\right) -m^2 \phi^{\dagger}\phi - \...
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1answer
28 views

Can I make the visible universe charged by firing short lived charged quantum black holes? [closed]

Imagine I fires a couple of negatively charged quantum black hole and each black hole would last probably a second or two before releasing enormous amount of pure energy, haven't I single handedly ...
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1answer
22 views

Why is anti-neutrino produced along side with electron in beta minus decay? [duplicate]

so i get that when there is an excess of neutrons, neutron is converted into a proton. An electron is produced to neutralise the +1 charge of proton. But why is an anti neutrino produced?
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1answer
45 views

Violation of Gravity as a Conservative force?

I was trying out a simulation about escape velocity to understand why direction has no impact on it here :http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/...
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2answers
62 views

A basic formula of 1D elastic collision derivation

I found the folowing formula for 1d elastic colition: $$v_1 +u_1 =v_2+u_2 $$ (v,u means the velocity before and after the collision respectively) I tried to derivate it from momentum and energy ...
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0answers
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Why is the decay $\eta'\rightarrow \gamma + \gamma$ suppressed?

So the $\eta$ meson's dominant decay mode is $\eta \rightarrow \gamma + \gamma$ whereas the dominant mode for $\eta'$ is $\rho+\gamma $ or $\eta + \pi+\pi$. Why does it not decay to two photons ...
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2answers
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Can I feed enough spin up electrons to a black hole to affect its angular momentum?

I was reading classical spin vs quantum field spin. I know spin in quantum mechanics is just a quantum number. But what happens if I try to intentionally feed many electrons all in the same spin state ...
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2answers
49 views

Conservation of momentum in different reference frames

Does the total momentum of a system in a given inertial reference frame have the same numerical value in other inertial reference frames?
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2answers
42 views

Conservation of Energy/Linear&Angular Momentum

If a ball hits a rod at rest at any position along the rod, the rod will be moving with the same linear velocity in each case. However, if the ball hits the rod away from its center of mass, the rod ...
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1answer
33 views

About Kepler's laws [closed]

Yesterday I was reviewing some calculations using Kepler's laws and when working the third law that relates orbital periods and distances, it is very normal that, given the parameters of some object ...
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1answer
44 views

Angular momentum w/ changing moment of inertia

A man of mass m1 is standing on a disk with radius R and mass M at a distance r The man starts walking around the disk with constant angular speed w1 and as a result the disk begins to rotate in the ...
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1answer
39 views

Centre of mass problem

I came across a problem where there was an ice and tray system. The system was placed in a gravity free hall. After the ice melted the shift in centre of mass was to be found. But the answer is 0. Is ...
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1answer
44 views

Using Hawking radiation to turn matter into antimatter and vice-versa

I asked this question on the astronomy stack exchange: https://astronomy.stackexchange.com/q/30477 and it was recommended to me that I instead ask it here. My question is related to the following ...
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1answer
56 views

How does orbiting work exactly? [duplicate]

First of all I'm not a physicist or anything and I don't know much about physics, so I'm sorry if this doesn't make sense at all :) And if possible please try to explain it with words instead of ...
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3answers
119 views

Can conservation of angular momentum be derived from energy consideration? [closed]

About the context of this question: A dramatic illustration of back and forth conversion of kinetic energy and gravitational potential energy is when a celestial body has an highly eccentric orbit. ...
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1answer
60 views

How is energy conserved in an elastic collision between two unequal masses?

In an elastic collision between two masses, if one mass is much heavier than the other, then the heavier mass will continue to move with same velocity while the lighter mass doubles its velocity. How ...
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1answer
52 views

How could I derive the Noether charge for a real scalar field?

I know for a (free) complex scalar field $\psi$ the Lagrangian is: $$ \mathcal{L} = \partial^\mu \psi^\ast\partial_\mu \psi$$ and that Noether's theorem from the $U(1)$ symmetry of the system gives a ...
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4answers
96 views

Collision of block and a spring system kept at rest

A body of mass $m$ moving with velocity $v$ collides with a system of 2 blocks of each mass $m$ connected by a spring of spring constant $k$. All the three lie on the same plane (1D collision). There ...
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1answer
104 views

What is a coordinate-free version of Noethers theorem? [closed]

What are some examples and derivations of some basic symmetries (not coordinate symmetries)? For example I remember a sufficient condition for being a symmetry of the lagrangian system is being an ...
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0answers
16 views

Reference request: Hydrodynamics for transport

Could someone suggest a good book/review that could be used to understand hydrodynamics of conserved quantities for simple systems (say for example to understand energy transport in a chain of ...
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1answer
120 views

Infinitesimal transformations that leave the action invariant

I have the Klein-Gordon Lagrangian for three scalar fields and I want to find three independent infinitesimal transformations that leave the action invariant. I suppose that these three ...
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2answers
56 views

What happens energy-wise if you put a gyroscope at the north pole?

I had an idea for energy generation: put a gyroscope at the north or south pole connected to a dynamo or similar. The idea was that when the gyroscope resists the Earth's rotation, it would drive the ...
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2answers
85 views

Parity in $\rho^0 \rightarrow \pi^0+\gamma$ decay

I am doing a homework question for $\rho^0 \rightarrow \pi^0+\gamma$ decay. It is given the $J^{PC}=1^{--}$ for the $\rho^0$ meson and that parity is conserved for this process. To calculate the ...
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6answers
7k views

Why doesn't Newton's third law mean a person bounces back to where they started when they hit the ground? [closed]

Why doesn't a person bounce back after falling down like a ball? If we push a person and he falls down then why doesn't he come back to its initial position. Although according to Newton's 3rd law of ...
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3answers
77 views

Momentum conservation: Will the cart move?

I know the law of conservation of linear momentum. I thought of the following situation. A block at some height is made to fall freely on a cart which is moving with a constant velocity. Will the ...