Questions tagged [conservation-laws]

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

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Collision of Rolling body

When a rolling body collides with a wall elastically,its torque is zero.But direction of velocity of particle is reversed.So will the angular momentum be conserved?
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Reversibility, Entropy and Information in Quantum Mechanics

I suspect that I harbor some misconceptions about these topics. A first item is that QM processes are time reversible. So, in principle, the entropy must remain constant. (??). A second item is the ...
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Conserved charges generate transformations

Focussing on classical mechanics of a point particle, WLOG since it captures the relevant information for field theory and generalises to the quantum case, how do we show -- in general -- that ...
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Verifying completeness of constants of motion

I can find constants of motion by looking at the null space of the Poisson Bracket operator $\{H, \cdot\}$ over a polynomial space by brute force with symbolic algebra (code). This scales terribly ...
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Prove the Total Mechanical Energy of the System is Conserved via Differential Equations [closed]

Consider the dynamics of a particle P shown: Particle in 3D space with Radius r Newton's second law states that: $$\frac{d}{dt}(m\dot r) = \mathbf F$$ where, $\boldsymbol{r}$ is the position vector ...
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Linear momentum of a system remains conserved, but with respect to which frame of reference?

I have studied that linear momentum of a system remains conserved. But i can't figure out with which reference of frame it is conserved. Is it conserved with respect to system reference frame or in a ...
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Do the balls keep moving forever?

In a hypothetical world where there is no friction, and all collisions are elastic would an object in lateral motion be perpetually moving and never come to a stop? Assumptions: Perfect vacuum: zero ...
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Conservative or non conservative convection

I want to describe the convection of a species $B$ by another species $A$. Meaning that $A$ is bringing $B$ with it, and there is no other velocity associated to $B$. So I have 2 conservation ...
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The impulse on a wall

If a ball was thrown to a wall, How could we calculate the impulse on the wall ? For example, if I throw a $2kg$ ball with $4m/s$, and it returned with the same speed but in the opposite direction, ...
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Is potential energy always inversely proportional to separation?

This is a question from Class 11 Physics NCERT. The answer is that only graph (v) is correct and the reason given here is that potential energy varies inversely with separation. But does that always ...
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How to show that translational invariance in $y$ of implies that it's an eigenstate of $p_y$?

Let us consider a particle on a plane with uniform magnetic field $B=B\hat{z}$, and hence with the Hamiltonian $H=\frac{1}{2m}(\vec{p}+e\vec{A})^2$. I am concerned with finding the energy eigenstates, ...
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Why bending or reorienting the wire does not change the validity of Kirchhoff's junction rule?

Kirchhoff’s junction rule is based on conservation of charge and the outgoing currents add up and are equal to incoming current at a junction. but Bending or reorienting the wire does not change the ...
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How did we know that relativistic momentum is conserved in the first place?

From what I understand, people historically did a lot of experiments (or just life experience) realizing that a fast moving object of small mass has the same quantity of "something" that a slow moving ...
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Conservation of linear momentum with mass defect

Suppose we have an insulating container, like a perfect black body, which absorbs all the radiation coming from a radioactive element placed in the center (or some equivalent process like matter ...
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How to check if a generating function produces an identity transformation without substituting the CT equations in the Hamiltonian?

In chapter 9, Goldstein ($3^{rd}$ ed.) includes a discussion and a few "trivial special cases" of Canonical Transformation which keeps the form of the Hamiltonian unchanged and named it Identity ...
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Deriving the Tsiolkovsky Rocket Equation using the rate at which the fuel burns

I'm trying to understand the derivation of the Tsiolkovsky Rocket Equation. $$\Delta v= \int_{t_0}^{t_1} \dfrac{|T|}{m_0 - t\Delta m}dt$$ The Wikipedia page ...
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Relative velocity, momentum conservation

I am having difficulty in conserving momentum for objects moving with relative velocities. For example: A block of mass $m$ with a semicircular track of radius $r$ rests on a frictionless ...
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How is linear momentum conserved in case of a freely falling body?

When an object is experiencing free fall, it has a constant acceleration and hence an increasing velocity (neglecting friction). Thus its momentum is increasing. But according to law of conservation ...
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Fields and Conservation of Energy

I am a bit confused about fields, like magnetic fields, and the conservation of energy. For example, when two magnets are attracted to each other, don't they "exhaust" the field? The moon that orbits ...
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Does an atom recoil when photon radiate?

Consider an atom in the excited state radiating a photon and goes to the lower energy state. But photons have a certain angular momentum, the momentum itself is not defined. In this case, will there ...
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I am following the book by Freedman and Van-Proeyen and this question is related to exercise 6.3. The supercurrent of a super Yang-Mills theory is given by $\mathcal{J}^{\mu} = \gamma^{\nu \rho} F^... 1answer 74 views Why is$\Delta^0$not decaying weakly? Given the lower decay I wonder why it happens this way. Wouldn't it be possible to decay via a weak process as well? 2answers 73 views Symmetry reason why magnetic dipole transitions are suppressed In the theory of light-matter interaction, electric dipole transitions between two atomic states of same parity are forbidden. This is because the Hamiltonian conserves parity. Is there a symmetry ... 0answers 63 views Rigid body dynamics exercise Yesterday I came across this exercise and I can't really find a way to get the correct answer. Exercise A rigid homogeneous rod that weighs$M=5\,\mathrm{Kg}$, has length$l=0.8\,\mathrm{m}$and a ... 1answer 32 views Energy threshold for photon I just read https://en.wikipedia.org/wiki/Annihilation#Examples and there is the popular$ e^+e^- \to \gamma \gamma $reaction described. Is it always possible to produce a$ \gamma $? A photon does ... 1answer 87 views Why is$J/\psi$not being able to decay into$\pi^+\pi^-\pi^0\$ (Ozi-rule)?

There are already (very similar) questions like What is the precise statement of the OZI Rule? and Why does charmonium (and phi mesons) not decay via quark and antiquark annihilation? but, to be ...