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

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148 views

Conservation of momentum and conservation of energy

I had the impression that conservation of momentum is valid only when we consider no loss of kinetic energy into heat etc. My tutor says that even if we have loss of kinetic energy as heat in a ...
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0answers
42 views

Is the conservation of probability in the Schroedinger's equation unique?

The Schroedinger's equation can be viewed as a diffusion equation with imaginary constants $a$ and $b$ satisfying, $$(1) \quad \Psi_t=a \cdot \Delta \Psi-b \cdot V(x,t) \cdot \Psi$$ However if $a$ ...
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1answer
263 views

Why is the decay of a neutral pion $\to$ electron-positron loop suppressed?

To my understanding the decay of a neutral pion into an electron-positron pair can only happen by the electromagnetic force and the mediation of two virtual photons in a triangle-diagram, so it is ...
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2answers
105 views

When a ball bounces on a wall at an angle, why does the angle of incidence equal angle of reflection?

When a ball is thrown to a wall without spin and bounces, supposing it's an elastic collision, why does the angle of incidence equal the angle "reflection" (relatively to the normal), why isn't it any ...
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0answers
54 views

Continuity equation - water mass conservation

I have a problem to understand a passage in the article "Modeling the dynamics of pressure propagation and diameter variation in tree sapwood" that you can find here: ftp://ftp.bgc.mpg.de/pub/outgoing/...
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26 views

How can intuitively guess what conserved quantities has the system that I am studying?

I'm taking a course in Classical Electrodynamics and in one problem my teacher introduced us to a triplet of fields ($\phi^a$) invariant under internal rotations, i.e. transformations like: $$\phi'^...
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0answers
48 views

Higher order versions of momentum? Can conservation principles be established and used? [closed]

Question Can higher order derivatives of momentum be useful in creating theories of dynamics if they have conservation principles? Even if they aren't needed, could it be done in theory? For instance,...
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1answer
78 views

Nail clippers and conservation of momentum

As I clip my fingernails using nail clippers, pieces of nails sometimes fly off violently, reaching the other end of the room. What's happening in terms of conservation of momentum, how are the pieces ...
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1answer
93 views

conservation of momentum? [closed]

At hyperphysics I got this image, with the same description in text as is in this image It says that when a massive particle (say $A$) moving with a velocity collides with an object having a ...
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1answer
62 views

Conservation of momentum always? [closed]

Consider two atoms A and B, with A at rest and B in motion towards A. As soon as B is in the atomic field of A, will it stop with its energy going into B or will it just move up to a postion where ...
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1answer
104 views

Newton's third law at the quantum level? [closed]

let's look at force at the atomic level to understand the newtons third law of motion. I'll use Helium atoms as an example. Now imagine we start with one atom HE2 stationary, and throw another atom ...
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1answer
67 views

Second Kepler's law implies that constant velocity

I was trying deduce that if we suppose that the planet's orbits are circular and de Kepler's secong law is true: A line joining a planet and the Sun sweeps out equal areas during equal intervals of ...
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1answer
106 views

Why doesn't the 9th ball move in the break in the nine-ball pool game?

In the game of nine-ball pool, we break the rack like shown below: In the break, we hit the 1st ball with the cue ball. Many people familiar with pool games say that if the rack is constructed ...
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2answers
47 views

Why can't a force between two particles exert a torque?

Let's say I have two particles which exert a mysterious force between them. Say they lie on the x-axis. Newton's 3rd law tells us that the force on the two particles due to the other must be opposite ...
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1answer
135 views

Mathematical proof of an electron can't absorb a photon [duplicate]

How can we mathematically prove that a free electron can't absorb a photon totally?
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2answers
167 views

Multiple conservation of momentum in one [closed]

I am in a Grade 12 Physics course and my teacher gave this question for bonus marks, to be solved with only highschool concepts. I suspect that this problem has to do with conservation of momentum, ...
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1answer
76 views

How does potential energy relate to kinetic energy on a rollercoaster?

This is appearantly how the potential and kinetic energy are in the different positions of the rollercoaster, if I am not mistaking. However I don't understand why the total energy is energy and ...
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1answer
56 views

Isolated system and external forces

I have problem of understanding fully "isolated system", if i imagine a car moving at a constant speed ,can I say that is isolated system although an interna force (car's engine) cancel out an ...
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2answers
145 views

Violation of the Newton's first law of motion in the movement of spacecrafts in the vacuum of space [closed]

Suppose a rocket at rest in space where there is a complete vacuum and no force to influence the rocket: In the left we have the rocket with a metal ball in it. After the ball is thrown out of the ...
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2answers
52 views

Why does current density changes but not current? [closed]

let us assume that we have a conductor with a specified resistance ( case 1 ) and a normal conductor ( case 2 ) as shown in the figure , and now we apply an external electric field with a battery on ...
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2answers
158 views

Why does Conservation of Momentum fails because of friction? [closed]

I did this experiment in my lab. We had two cars with wheel connected by a string. One car has a spring that can be triggered by a switch at the top of the car. (Frictionless surface and light string$ ...
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0answers
16 views

Relativistic particle decay in ZMF [closed]

This is a problem that I feel is harder than I am making it but say you have: A particle at rest of mass $M$ decays into two smaller particles of masses $m_1, m_2$ where $m_1 \neq m_2$ and $M > (...
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3answers
160 views

Conservation of Momentum

Conservation of momentum can be derived during collisions by using Newton's laws of motion. But in other cases, do we simply take it like an axiom ?
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108 views

Conservation of linear momentum (classical mechanics and special relativity)

How did Newton deduce the law of conservation of linear momentum? Can it be derived only by Newton's laws, or does it follow from practical experiments? If the law of conservation of linear momentum ...
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1answer
98 views

Are there conserved quantities in field theory which don't arise from Noether's Theorem?

In some QFT texts one writes down the number operator $N$ for free theories, such that when acting on an $n$-particle state $|n\rangle$ we have $$N|n\rangle=n|n\rangle$$ In free theories this is a ...
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129 views

Conservation of angular momentum while sitting on a spinning chair

Today my friend was sitting on a spinning char. By moving his top part of the body left to right and his bottom part of the body the opposite he managed to spin. As I understand Conservation of ...
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1answer
115 views

Conservation of angular momentum during rolling

A disk having initial angular velocity $\omega$ is gently placed on a rough horizontal surface. What is the angular velocity of rotation when pure rolling starts? I've tried applying conservation of ...
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0answers
44 views

Gravitons, photons and conservation of momentum

How can gravitons be emitted from a mass to cause an attractive force to another mass? The same question could be asked of attractive e-m forces as well. Don't these violate the conservation of ...
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1answer
47 views

Help with my bouncy ball lab (I know the factors just not how to approach them) [closed]

In my physics lab we need to determine the factors that account for the energy "loss" during a high bounce ball bounce. I know that energy is "lost" (not really) to heat, air resistance, and sound. ...
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2answers
82 views

Inelastic collisions

During inelastic collision the kinetic energy does not remain constant. In many texts, I have seen it is because there is loss in energy from the system. If there is loss in energy, then how can mass ...
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1answer
77 views

Proving that Killing form contractions with geodesics are constants of motion

I want to prove the fundamental theorem of Killing forms, namely that $$\frac{d}{d \lambda} \Big( \frac{d P^{\mu}}{d \lambda} \xi_{\mu}(P(\lambda)) \Big) = 0 $$ If $P(\lambda)$ is a Geodesic curve,...
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1answer
71 views

Einstein tensor as a conserved current?

As is well-known, the ``traditional" conserved quantities (energy, momentum...) are Noether currents whose conservation depends on the existence of various Killing fields in Minkowski space. In ...
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1answer
81 views

Why particles with certain properties can't exist

This is inspired by a recent post on why a free electron can't absorb a photon, though my question below is about something considerably more general. The argument in the accepted answer goes (in ...
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2answers
201 views

Why are gravity and spring-force conservative forces?

I know that the amount of work does not depend on the followed path but on the position when speaking about conservative forces. But could someone please prove/explain why exactly those two are ...
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1answer
62 views

Conserved currents from Noether's theorem

I'm not sure if I understand the concept correctly. Given an infinitesimal transformation $$\phi \rightarrow \phi + \alpha \Delta\phi$$ the change in the Lagrangian density $\mathcal{L}(\phi,\...
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1answer
72 views

Annihilation process and photons [closed]

Why should two photons produced by the annihilation process move in opposite direction? I know you would say for the conservation of momentum but why can't they move in the same direction, I want to ...
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3answers
118 views

Why can't we define a potential energy for a non-conservative force? [closed]

We could define potential energies for non-conservative forces too and then we could conserve it with kinetic and potential energy as we know it. But no one does that. Why is this? Please explain. Any ...
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1answer
239 views

Applying conservation of energy on a springs/projectile motion problem

My physics teacher proposed a hypothetical problem to our class as we are nearing the end of the Work-Energy Unit (that will in all likelihood show up on the test). I will probably have various ...
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0answers
62 views

How can we solve 2D rigid body collision? [duplicate]

I know that usually collision with velocity collinear can be solved by simultaneous equations of both conservation of energy and linear momentum. But my question is when 2D velocity is encountered, we ...
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1answer
225 views

Conservation of linear momentum and velocity of a system (damper and spring in a series)

This example is from a book on dynamics. Let us consider the system above formed by two blocks (each of mass $m$) connected by a linear damper and spring in a series. They slide without friction on a ...
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3answers
555 views

What does it mean to say work is path-independent when pushing an object in different directions?

If I apply a straight upward(perpendicular to ground) force against gravity of $5\ \mathrm{N}$ and lift an object "A" 10 meters, then the work done is: $$ W = F \times S = 5\ \mathrm{N} \times 10\ \...
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Does a planet's orbital angular momentum affect its rotational angular momentum?

For example: If the moon was closer to the earth, assuming the orbital momentum was conserved and not worrying about earth's rotation, would the moon's rotation rate be effected?
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3answers
108 views

A moving bus suddenly stops. Is its momentum destroyed?

Absolutely not but how does momentum transfer to surrounding (ground, air particles)?
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72 views

At what speed is the conservation of angular momentum carried out?

If the sun suddenly slowed down would pluto's orbit immediately speed up? If so, then isn't the information pertaining to the sun's angular momentum change being carried to Pluto faster than light? ...
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What symmetry is associated with conservation of Lipkin's zilch?

The 'zilch' of an electromagnetic field is the tensor $$ Z^{\mu}_{\ \ \ \nu\rho}=^*\!\!F^{\mu\lambda}F_{\lambda\nu,\rho}-F^{\mu\lambda}\,{}^*\!F_{\lambda\nu,\rho} \tag1 $$ given in terms of the ...
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1answer
52 views

How do I solve for $v_2$ where $mv_1^2 + MU_1^2 = mv_2^2 + M U_2^2$ and $MU_1 - Mv_1 = MU_2 - mv_2$ by eliminating $U_2$?

I was trying to solve the head on collision slingshot problem where the rocket moving with speed $v_1$ approaches a planet which is moving with speed $U_1$. I wanted the final speed of the rocket ($...
12
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1answer
300 views

How do higher-order optical chiralities look like?

The optical chirality of the electromagnetic field is a conserved quantity, analogous to the energy density, linear momentum density, and angular momentum density, which describes how chiral the EM ...
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1answer
303 views

Angular momentum paradox with 2 identical gears

Consider two identical gears touching each other. The system is friction less. One has a handle that you use to apply a torque on the entire system. If you turn the handle, there will be a non-zero ...
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87 views

A mysterious conserved quantity for a central potential

In teaching a course in classical mechanics and I have come across (from my predecessor) a to me mysterious conserved quantity. We are considering a gravitational (or electric) potential with the ...
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207 views

How is angular momentum conserved

from a classical perspective, what is it about angular momentum fundamentally that means it has to be conserved? Surely if I have a rod about a fixed axis and a moving particle hits the end it will ...