# Tagged Questions

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

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### How does conservation work in wormholes?

My questions might seem naive as I haven't plunged into the mathematical descriptions, but wormholes are interesting even if I don't actually know them :) First of all, is wormhole making two ...
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### Need help solving a task - laws of conservation (leptons, baryons, charge)

We got a task from our teacher, a multiple choice task; and here it is: Which of the following reactions is correct, considering the conservation laws? Note: We have only learned to use the ...
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### Conserved quantities and total derivatives?

I am having a bit of a crisis in understanding of the physical meanings of total derivatives. When a quantity $\rho$ (be it a vector or a scalar) is said to be conserved, then (mathematically) ...
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### Is energy conserved in general relativity? Does $\nabla_aT^{ab}_{\rm matter}=0$ represent the conservation of energy and momentum?

For example, the radiation dominated cosmology, the energy density of radiation is proportional to $a^{-4}$ and the volume is proportional to $a^3$, where $a$ is the scale factor. So the total energy ...
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### Collision of Discs and Snooker Kicks

I woke up this morning thinking about spinning discs. Could someone verify whether my reasoning below is correct? Problem 1 Suppose have two identical uniform discs constrained to move in a plane. ...
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### Why is energy conserved? [duplicate]

From basic principals, how does one prove that energy is conserved? Or a little more specifically - Why does this hold: $$\Delta \mbox{ PotentialEnergy} + \Delta \mbox{ KineticEnergy} = 0$$ Or, for ...
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### What are the general solutions to a hard sphere collision?

Surely someone has found the solutions to the hard sphere collisions (in $n$ dimensions) of two bodies of mass $m_1$ and $m_2$, respectively--that is the resultant velocities (or momenta) of the two ...
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### Pion decay in particle physics

I'm taking a particle physics course and we're using Perkins Introduction to High Energy Physics as the text. I am looking at problem 1.7. It asks whether $$\pi^0\rightarrow e^- + e^+$$ is allowed or ...
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### Lepton number conservation in standard model

Why is it said that in standard model lepton number is conserved? How do I know that Lepton number is an abelian charge? Why is this conservation not as sacred as electric charge conservation. How ...
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### What is the symmetry associated with electric charge conservation [duplicate]

Is there a kind of symmetry that yields the conservation of charges? and if so , how it works for both type of charges? Electrical Charges
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### Recoil from a shotgun

First of all I'd like to point out that I'm a complete layman when it comes to physics and I apologise if this isn't the correct forum for this. If I were to sit on an office chair on a laminated ...
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### Could elementary particles join to create bound states?

I recently had question, can 2 elementary particles be "attached" together using Strong and weak nuclear forces to create a bound state. For example can Electron and some other stable elementary ...
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### Energy conservation limited by uncertainty principle

The way I learned it from practicing Fourier analysis and signal processing besides quantum mechanics, is that Energy conservation cannot be achieved in short time scales, and that limits energy ...
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### What is the exact relation between $\mathrm{SU(3)}$ flavour symmetry and the Gell-Mann–Nishijima relation

I'm trying to understand how the Gell-Mann–Nishijima relation has been derived: $$Q = I_3 + \frac{Y}{2}$$ where $Q$ is the electric charge of the quarks, $I_3$ is the ...
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### Energy/Momentum required to deflect earth from its orbit

This question occurred to me when thinking about the firepower people have on earth. How hard is it to change the characteristics of the orbit of the earth when an explosion happens on its surface, ...
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### Thrust to Weight ratio in Space with an off set CoM

With regards to this thread, Thrust center in space My question is, if the thrust to weight ratio was increased so that it was much higher than the weighted mass of the sphere (ship), would the ...
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### How are symmetries defined mathematically? [duplicate]

I have started working on differential geometry very recently. I am little bit familiar with mathematical concepts such as manifolds, differential forms and associated concepts. As I was speeding ...
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### Oscillations and Majorana Neutrino

In neutrino oscillations, neutrinos can convert from one flavor to another. This implies individual lepton number is not conserved. Doesn't it also imply that, if the neutrinos have mass, the mass ...
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### Regarding derivation of Probability Current

The question for the full derivation of Probability Conservation -> Probability Current was already asked here: Probability current. I apologize for not retyping it out, but it's already beautifully ...
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### A paradox to Lenz's law

I have read that in simple words, Lenz's law states that: The direction of current induced in a conductor is in such a fashion, that it opposes its cause. This validates law of conservation of ...
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### Relative Velocities and Conservation of Kinetic Energy

An object of mass m moves with velocity $v$ towards a stationary object of same mass. Impact is an elastic collision. $v_1$ is the velocity after impact of the mass originally moving $v_2$ is the ...
502 views

### How do I calculate stopping power?

Malcolm Gladwell made a claim in a recent talk that a sling with a stone going at 30m/s has the same stopping power as a .45 calibre handgun. How would I verify whether or not this claim is true - ...
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### Equal and opposite reaction - forces coming in pairs - or just common sense?

I've never understood why the conservation of momentum law is taught in schools as "every force has an equal opposite reaction". To me a gun's recoil is self explanatory; the explosion sits ...
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### Change in Angular momentum of a spinning superball

I throw a superball towards the ground and when I do I intentional induce a spin on the ball so that the ball is spinning towards me on the descent. As soon as the ball hits the ground and comes back ...
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### Orbital angular momentum selection rules for three identical particles

I'm trying to figure out if there are selection rules for the total orbital angular momentum for a system of three identical particles, say bosons. For two identical bosons one can argue that the ...
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Encountered a problem that involves impulse while studying for my exam and I'm not sure how to even approach it. I know that momentum is conserved, but I'm not sure how to relate that to avg force. ...
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### Is there any way to annihilate matter without the use of anti-matter?

Is there any way to annihilate matter without the use of anti-matter? And vice versa? I mean, for example is it possible to totally convert the mass of a proton into "pure energy" without use an ...
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### Is this a valid proof that the four-current is conserved?

The four-current of a particle moving along a worldine $X^\nu(s)$ is defined as $$j^\mu(x^\nu) = ec \int u^\mu(s)\, \delta^4(x^\nu - X^\nu(s)) \, ds$$ So here's my proof that this is conserved: ...
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### Continuum limit for solid mechanics

Is there a rigorous derivation of the limits for continuum properties in solid mechanics? For instance, the stress-strain relationship may be linear for large samples (the slope being the Young's ...
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### Movement in outer space via Newton's law of every action has an equal and opposite reaction

What is more effective for travel in outer space ignoring all other factors like air radiation etc. I have a 10 kg bag of rice would I travel faster throwing the whole bag at once or throwing a grain ...
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### Is there such a thing as instantly stopping?

I'm sorry if this is a stupid question, but I've never taken a physics class and I was curious about something. But anyway, my question is, is there such a thing as instantly stopping? For example, if ...
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What's a good book (or other resource) for an advanced undergraduate/early graduate student to learn about symmetry, conservation laws and Noether's theorems? Neuenschwander's book has a scary review ...
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### Does the action and Lagrangian have identical symmetries and conserved quantities?

From the book Introduction to Classical Mechanics With Problems and Solutions by David Morin, page 236 states: Noether's Theorem: For each symmetry of the Lagrangian, there is a conserved ...
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### Is the canonical momentum conserved when a particle moves in magnetic field?

Here is a question about the canonical momentum that I had asked some days ago, but I still have one point that I am not understand. Considering a particle moves in a magnetic field with charge $q$ ...
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### Why is electric charge conserved?

We have long been taught that electric charges are neither created nor destroyed. But somehow it is okay to destroy two oppositely charged particles at once ! Why is that so? Let's just take a look ...
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### What are the correct initial conditions for the moon (in a simulation)?

So I've modeled the interactions between the sun and all the planets (and the interactions between the planets) using Verlet integration. I've used data from Wikipedia for masses, distance from the ...
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### No valid Feynman diagram for processes

This will likely be easy for anyone experienced in particle physics, but I'm not. I'm asked to explain why it is impossible to construct a valid Feynman diagram using Standard Model vertices for the ...
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### Quantum explanation of Newton's Third Law of Motion

Newton's law states that for every action there is equal and opposite reaction. This law explains how rockets fly in space and accounts for the the majority of the lift action generated by a ...
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### How to apply conservation of angular momentum with a shock? [closed]

I got this tricky question, need help. A uniform rod of mass $M$ and length $L$ is attached to an axis at its top, a bullet with mass $m$ traveling at speed $U$ (horizontal) hits the rod at $2L/3$ ...
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### Angular Velocity after a frictional impulse [duplicate]

I am modelling 2D physics collision into simulations. In Physics for Game Programmers, Grant Palmer book, the velocity Vn1 after collision is mentioned to be independent of the friction coeff. ...
516 views

### Rotational invariance and operator-squares

My mind is drawing a blank right now. In systems with spin and orbital angular momentum, I know that rotational invariance implies that $[H, \mathbf{J}]=0$ where $\mathbf J=\mathbf L+\mathbf S$. But ...
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### Why must SUSY be broken?

Background One usually claims that supersymmetry must be spontaneously broken. The reasoning is roughly the following: Since $M^2=P^{\mu}P_{\mu}$ is a casimir operator of the supersymmetry algebra, ...
I know this question is asked here a lot, but I just had to ask this to finalise the concept. When a system lets say a rod of length $L$ and mass $M$ is rotating with angular speed $omega_1$ its ...