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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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About “conserved quantities” in a diffeomorphism-invariant theory by Wald and Zoupas

In this work, Wald & Zoupas developed a framework to define the "conserved quantities" in a diffeomorphism-invariant theory using the covariant phase space formalism. Let $\xi^a$ be a vector ...
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3D perfectly elastic collision between two points

There is a high probability, I think, that this question is a duplicate of some other question ... but to may knowledge, it hasn't been posed in this exact manner: Assume we have 2 points, $P_1$ and $...
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Charge of multiverse

The charge of the universe is said to be zero. The most general argument I hear about this is that: since there was no net charge before the universe there must not be any after. Hence the universe is ...
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Is Smoluchowski equation for aggregation conservative?

The Smoluchowski equation for the aggregation with no breakage can be written as $$\frac{dn_k}{dt}=\frac{1}{2}\Big(\sum_{i+j=k} n_i \beta_{ij}n_j\Big) - n_k \sum_{i=1}^N \beta_{ki} n_i$$ where $n_k$ ...
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How to know what type of diagram contributes to a two-to-two process?

There are 3 types of diagrams that can contribute to a two-to-two process; the $s$-channel, $u$-channel and $t$-channel. How do I know what diagrams can contribute to a process? I know that in QED, ...
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Number of collisions with block [closed]

The problem is given in David Morin's Classical mechanics. Here is the screenshot: Now, I jumped to solve part b of question. To find the number of bounces,we note that mass losses momentum of -2mV ...
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Derive conservation laws for the energy momentum tensor, $T μν$ in coordinates $(τ,x,y,η)$

How can i derive this relation? i need simulation Transverse momentum spectra of π+ and K+ , but i do not know what discretized this tensor coupled partial differential equations`]1
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3answers
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Problem to solve momentum (Classic Newton mechanics) [closed]

First of all, I want to apologize for my English I'll try to explain my self clearly. I want to share one question about Newtonian mechanics from my homework, of which I can't understand the logic ...
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366 views

Still Confused about Linear Momentum in a Circle

A point mass with mass $m$, distance $r$ from circle and constant tangential velocity and constant angular velocity is swung around a circle. ($p$ is linear momentum) Angular momentum is radius x ...
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3answers
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Conservation of Momentum within Conservation of Angular Momentum

$L=r \times p$ where $L$ is angular momentum, $r$ is radius and $p$ is tangential linear momentum. Using a generic example of a skater spinning on ice with no friction while being stationary, ...
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Why can a particle decay into two photons but not one?

I recently read an old physics news about the Higgs boson where it was observed to decay into 2 photons and I was wondering why it wouldn't have decayed into a single photon with the combined energy ...
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Which specific law of physics is broken by the “double jump”? [closed]

So, there's a cliche in computer gaming known as a "double jump". This can be described as: A character jumps from a solid surface, and then is able to extend their jump by carrying out the jump ...
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Conservation of quantum information in mixed states quantum secret sharing (QSS) schemes?

Consider a $((3,5))$ pure state quantum secret sharing (QSS) scheme. For instance this paper: arXiv:quant-ph/9901025. If I divide any 5 shares to two sets then allways one of those two set are ...
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1answer
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Continuity equation from the Friedmann equation

I know that one can derive the continuity equation: $$\dot{\rho}=3H(\rho+P)$$ from the Friedmann equations. $H^2=\frac{8\pi{G}\rho}{3}$ and $\frac{\ddot{a}}{a}=\frac{4\pi{G}\rho}{3}(\rho+3P)$ But ...
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1answer
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Is outside force needed to conserve angular momentum?

Franklin Hu has a good experiment here. Franklin uses the correct formula for angular momentum; and he realizes that the Linear or Newtonian velocity must double if angular momentum is to be ...
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Noether's conserved charges: why are there not “spatially” conserved charges? [duplicate]

Noether theorem implies that there is a conserved current $j^\mu$ for every continuous symmetry of the action, i.e. $$\partial_\mu j^\mu=0 $$ to each conserved current we can associate a conserved ...
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Can a free particle ever emit a particle?

I have found this question here Can a free particle absorb/emit photons?, along with other resources that show a free particle cannot emit a photon (in a vacuum). Now, I am 90% sure it does, but ...
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What happens to the quantum information of a particle and an antiparticle when they annihilate?

I understand that the quantum no-deleting theorem dictates that it's impossible to delete quantum information, so what happens to the quantum information of a particle and an antiparticle when they ...
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6answers
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Why does shooting a handgun accelerate a bullet to deadly speed without injuring the gun user's hand?

Momentum is defined by the product of mass and velocity. Now a projectile out of a gun has to have high velocity to penetrate a human body, as its mass isn't significant. But to reach this velocity, ...
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1answer
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When atom absorb energy, how does the nucleon know? [duplicate]

Imagine a photon hits a hydrogen ion and is absorbed, how do the nucleon of the hydrogen knows and start to accelerate? Any difference between this and hydrogen ion in a electric field?
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1answer
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Where do I go from here to show that linear momentum is conserved under all instances of translation symmetry?

I've worked through a simple derivation of symmetries implying conservation laws from an invariant Lagrangian. Namely a quantity $Q$ is conserved in the equation below, where $i$ is a degree of ...
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1answer
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Inelastic momentum question: How to calculate velocity before the collision? [closed]

EDIT: NEVERMIND! MY APOLOGIES, I DID NOT READ THE QUESTION PROPERLY. I DID NOT SEE THAT IT SAID THAT THE GUY WAS STATIONARY. I was doing a practice physics question and stumbled upon a question ...
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0answers
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Black hole and lepton number conservation [duplicate]

Can falling of an electron into a black be treated as a process in which energy, charge, spin (angular momentum) and lepton number of the system are conserved? Imagine a small volume of space that at ...
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3answers
350 views

Angular momentum in different points

I have a question about angular momentum: Is it possible to have a system where angular momentum is conserved relative to 1 point,but not conserved relative to another?
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Does conservation of angular momentum break conservation of momentum?

Say we have a spinning ring of mass $M$, rotating at $W_0$, at a radius $r$ from some pivot point. This ring has massless spokes extending out to a length of $2r$. From this, we can calculate the ...
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1answer
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Where does the momentum go when atom absorb a photon? [duplicate]

Imagine an electron around an atom absorbs a photon and becomes excited, it has now jumped to a higher orbital. At this point in time, where does the momentum of the photon goes?
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3answers
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Is physics the same everywhere in the universe, what if it isn't? [closed]

So, my friend the other day brought up an interesting topic, does physics need to be the same everywhere? He asked, how do we know the universe doesn't, say, simplify itself, far away from us? For ...
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1answer
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Can we abstractly deduce the $L^2$ is conserved assuming only rotational symmetry of Hamiltonian?

Here $L^2$ is defined as $$ L^2=L_x^2+L_y^2+L_z^2 $$ representing the observable of the magnitude of the angular momentum. There are a lot of proofs showing the $z$-projection of the angular ...
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5answers
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Violating Newtons First Law!

Suppose you are inside a very large empty box in deep space , floating ( i.e not touching the box from anywhere initially).The box is at complete rest. Now you push the box forward from inside. Now ...
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2answers
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Conserved charge: partial or total derivative?

I want to obtain some clarification on the concept of Noether charge. Given conserved current $J^\mu$ e.g. in free scalar field theory in $(n+1)$ dimensional Minkowski spacetime $M$, i.e. $\partial_\...
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0answers
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How to relate $\gamma^5$ to the spacetime volume form? In regards to axial current anomaly

In playing with gamma matrices of the $\mathcal{\mathscr{C}}l_{1,3}(R)$ variety, it's not uncommon to hear allusions to $\gamma^5$ being related to the volume 4-form. To illustrate the similarities: $...
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Does the total kinetic energy change during an elastic collision?

If two balls of same mass with speed $v$ and $-v$ undergo an elastic collision, the kinetic energy will be the same after the collision as before. However, during the collision, does it also remain ...
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Are galaxies rotating faster or slower over time?

Independent of the discussion around flat rotation curves and dark matter, is there something known about the change of the rotation curves over time?
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2answers
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Newton's cradle: why not twice the balls at half the speed? [duplicate]

For "Sinterklaas" (Dutch holiday) I got a Newton's cradle. I understand the basics of it, but a couple things bother me. If I drop one ball, one bounces off. If I drop two, two bounce off. Why, if I ...
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1answer
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Santa and conservation of momentum. How is it possible that conservation of momentum doesn't hold here?

Santa wants to deliver his presents but his reindeers are on strike. In order to still be able to get somewhere he decides to sacrifice some of his presents to gain speed. He decides to throw them ...
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1answer
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Conceptual question on volume current density and derivation of the continuity equation

The expression for the volume integral of the volume charge density is $\int_{V} (\nabla \cdot\vec{J}) d\tau = -\frac{d}{dt} \int_{V} \rho d\tau = -\int_{V} (\frac{\partial \rho}{\partial t}) d\tau$...
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2answers
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Are there two action-reaction pairs operating in a rocket

I'm confused about the action-reaction pair in a rocket. is it 1 The action of the explosion of gases in the combustion chamber on the rocket, and the reaction of the rocket on the gases, or 2 The ...
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1answer
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How does a rocket obey Newton's third law? [duplicate]

How does a rocket push against its exhaust? Does the reaction come from the rocket nozzle or as a result of the imbalance of forces in the combustion chamber?
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1answer
69 views

Why aren't coordinates induced vector fields always Killing fields?

We have that $$ L_K g_{\mu\nu}=\nabla_\mu K_\nu + \nabla_\nu K_\mu$$ A vector field $K$ is a Killing field if $ L_K g_{\mu\nu}=0$, but consider the coordinate induced vector field $\partial_\alpha$, ...
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0answers
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Question about the concepts of Noether charge and Noether current

I read that a noether current occurs when the lagrangian assume vector values. Well, what are noether current and noether charge in comparison to elementary classical mechanics notions of Noether's ...
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1answer
34 views

How does elastic potential energy translate into kinetic energy and momentum?

Let's assume there's a spring with 10,000J of elastic potential energy placed between two objects with a mass of 10kg each. Like this: 10kg< 10,000J >10kg I assume that the energy would be ...
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4answers
144 views

Why Newton's 3rd law exists? [duplicate]

We know that there is always an equal and opposite reaction for an action along the line joining the particles exerting forces on each other, but it is a law we have to follow? Why does this law exist?...
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1answer
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In the case of the Dirac Equation,what forbids the free electron to absorb a photon? (electron magnetic moment)

It is straightforward to show from relativistic kinematics that a free electron cannot absorb a photon, as shown in this previous thread. However, It is also known that using the Dirac equation, you ...
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What does it mean if the Lagrangian density has explicit spatial dependence?

First off, I have seen this post here which asks seems to ask my question, but it is not properly answered. If the Lagrangian has explicit time dependence, then the total energy, and Hamiltonian, is ...
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5answers
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Can momentum not be conserved in an inelastic collision?

Consider an inelastic collision in which a ball B of light mass moves towards a ball of clay A and ultimately sticks to it. Suppose that the whole system (A+B) comes to rest. Every book on Newtonian ...
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3answers
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Cat falling from a tree

When a cat or any body falls over to the ground, how is momentum conserved? I was working on a problem of a cat falling on top of a skateboard, and the system travels together with a new velocity. ...
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1answer
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What is a 'system'?

Is physics a full game of observation and after observing we frame law??Is it really true? My second query is what exactly is a system? When I was studying laws of motion i felt that system is ...
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1answer
32 views

Utility of work and energy

Why do we study the scalars such as work and energy if we know how to solve daily classical mechanics problem with standard mathematics? My second query is about the main motive of defining ...
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2answers
63 views

Vacuum thrust, and how it is proven

Let’s talk about “Vacuum Thrust”. Simply put, thrust that is exerted in a vacuum. Space is a vaccum correct? So therefore there is no matter is space, it is just empty space. Thrust is measure in ...
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1answer
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Fluid mechanics

i have seen the following equation of continuity for fluid dynamics for compressible as well as incompressible fluids $\rho_1V_1A_1 = \rho_2V_2A_2$ and it is derived from conservation of mass ...