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

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Conservation of generalized energies in Newtonian and Relativistic Systems

I was considering the following problem: In a closed system, it is assumed that mass, momentum and energy is conserved. If we label the total mass of the System $M(i)$ at a time i, the total Momentum ...
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2answers
82 views

Force of a Train

Imagine that there are two trains and the first train is twice as long as the second train. They have the same mass per unit length and they are traveling at exactly the same speed. If the first ...
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1answer
56 views

Prove the relation between relative velocity ,momentum and energy

In Chapter 8 of F.Mandl's book Quantum field theory, during the derivation of the differential cross section, the following relation is used: $$E_1E_2v_{rel}=\sqrt{(p_1p_2)^2-m_1^2m_2^2} \,\, ,$$ ...
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1answer
75 views

Total divergence term and corresponding Feynman Diagram

A total divergence term added to the Lagrangian doesn’t affect the action because the integral of a total divergence vanishes. But if one attempts to derive the Feynman rules from the Lagrangian with ...
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3answers
64 views

Why doesn't an object that collides with one that is at rest just do a 180?

Newton's third law says "for every action, there's an equal and opposite reaction." So why is it, say, that when a pool/billiards ball hits the wall of the table, the ball doesn't just turn around?
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Conservation of momentum when rain pours into a wagon

Suppose a wagon is moving at constant velocity on a friction-less surface, and rain begins to fill the wagon. The net force on the wagon is zero, so momentum is conserved; as the mass of the wagon ...
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1answer
45 views

What is the motion of a balloon with a person climbing on a ladder?

Suppose you had a balloon of mass $M$ in the atmosphere at rest relative to the ground, with a ladder attached reaching towards the ground. A person (of mass less than $M$) begins climbing up the ...
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3answers
42 views

Domain of different types of collision?

Can anyone tell me why elastic collision occur between atomic particles? inelastic collision occur between ordinary objects? perfectly inelastic collision occur during shooting? ...
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2answers
59 views

Playing catch in space?

I was thinking about this on the bus (exam this week :P). What would happen if two people played catch in outer space? Lets say person A and B are in space at rest, A throws a mass m at B, what ...
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2answers
51 views

Why is there conservation of kinetic energy in elastic collision and not in inelastic collision?

Why is there conservation of kinetic energy in elastic collision and not in inelastic collision? What would be the difference that lead to conservation of kinetic energy in elastic collision and not ...
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0answers
43 views

Constant quantity associated to symmetry [closed]

I'm attending a subject in theoretical mechanics and we saw this fact that bugged me a little. It's by the way referenced in: John R Cary, Lie transform perturbation theory for Hamiltonian ...
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2answers
39 views

Does constant velocity of center of mass imply linear momentum is conserved?

I know that if momentum is conserved for a system, you can derive that the velocity of the center of mass of that system is constant. I was wondering if the second condition also implies the first: if ...
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1answer
64 views

Origin of momentum. Noether's theorem

My professor talked about Noether's theorem and how translation is the origin of momentum conservation. But why is it not velocity that is conserved but mass times velocity. And on the same note why ...
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16 views

Locus of a moving mass point

Two very small mass particles $m_1$, $m_2$ are connected by a $2l$ long, infinitely soft and inelastic thread without mass. The initial condition of the system before being freely released is as in ...
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4answers
152 views

Why is $M\frac{dv}{dt} = v_{rel} .\frac{dm}{dt}$ correct and $(M - dm)\frac{dv}{dt} = v_{rel} .\frac{dm}{dt}$ wrong?

Newton's 2nd law of motion can't be applied for mass-varying systems. Another force, known as Thrust must come to play. It can be measured using law of conservation of linear momentum. ...
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312 views

Entire Universe's Momentum

I was thinking about the definition of the conservation of momentum, which says that momentum is conserved unless outside forces are acting on the system, and I was wondering that if the system is the ...
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0answers
33 views

If weak isospin is not conserved in time, what does the Noether theorem tell us?

As far as I understand weak isospin is only conserved in interactions but not as time evolves. Nevertheless, we get from Noethers theorem, because of global $SU(2)$ invariance a conserved quantity ...
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1answer
37 views

Why is rho to two pions not allowed? [duplicate]

Why is the $\rho^0 \rightarrow \pi^0 + \pi^0$ decay not allowed? I have seen this question but I am not satisfied with the answers. The $J^{PC}$ of the $\rho$ and $\pi$ are $1^{--}$ and $0^{-+}$ ...
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2answers
87 views

A student of mine wants to build a Gyroscopic Inertial Thruster

I work as a tutor with students in their first three semesters. One of my more eager students is obsessed with this thing: A Gyroscopic Inertial Thruster Here is a Wikipedia Article on that. I was ...
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0answers
58 views

Throwing a baseball against the back of a car [closed]

We, some fellow students and I, are trying to figure out what to. But we simply do not know which formula to use at question a. We found the following: $\Delta P=(2mv_{ball,i}M)/(m+M)$ Could someone ...
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2answers
65 views

Mathematical subtlety in a physics problem

A wagon of initial mass $M$ is moving with an initial velocity $V_o$. Now sand starts accumulating on this wagon, increasing the total mass from $M$ to $M+s$. We have to find the final velocity of ...
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5answers
84 views

Relation between potential energy and forces

Can you also explain why can't we define potential energy corresponding to a non-conservative internal force? Non-conservative forces are those which don't depend on the initial and final states but ...
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0answers
58 views

Why is only the third component of weak isospin used as a conserved quantity?

Using Noether's theorem \begin{equation} \partial_0 \int d^3x \left(\frac{\partial L}{\partial(\partial_0\Psi)} \delta \Psi \right) = 0 \end{equation} we get three conserved quantites $Q_i$ from ...
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4answers
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If angular momentum is conserved, what's wrong with this scenario?

I understand that, in a given system with no external torques applied, angular momentum is conserved. However, consider the following situation: Let's say the earth revolves around the Sun in a ...
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6answers
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Can a car steer on a frictionless surface?

Do the front tires of a car act like gyroscopes, such that a car could steer on a frictionless surface?
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2answers
123 views

What motion can emerge in a still universe? [duplicate]

I consider a very large non homogeneous cloud of dust in isolation. No external gravitational interactions. It is initially completely still and start collapsing under the its own gravity. Can the ...
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3answers
185 views

What is the origin of spin of celestial objects?

In an older question from June 2011, Why does each celestial object spin on its own axis?, apparently revived by the system, a user is asking about the origin of the rotation of celestial bodies. The ...
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2answers
81 views

Momentum conservation in an electromagnetic system?

Suppose that I have two charged particles in the configuration below. Let us assume the following: We apply a constant force $f$ to the the bottom particle so that it has a constant acceleration ...
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1answer
58 views

How positron and electron annihilate forming photons? [duplicate]

Electron is a particle with momentum $p$ and it spins up. Positron is its antiparticle having momentum $-p$ and it spins down. "A positron is an electron travelling backwards in time" said by Feynman. ...
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3answers
80 views

Is linear momentum conserved in an isolated system always?

Consider a isolated system of particles. If no external force acts on this system, the momentum of the system should remain constant. If the isolated system of particles represents living ...
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4answers
215 views

Why Does Angular Velocity Increase as Radius Decrease?

Suppose a child were to ask you why a tetherball (picture below) seems to speed up as it wraps around the pole. How would you explain this to them? Certainly you wouldn't say something like, "Angular ...
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5answers
360 views

Why is there no relation between kinetic energy and momentum in collision of two bodies? [duplicate]

The statement that baffles me: During most of the collisions, part of the kinetic energy evolve as heat, nevertheless momentum is still conserved. Ok, the statement may be true. But what ...
2
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1answer
49 views

momentum and energy of the rods

I have two electromagnets and I attach them to two rods(same mass) such that, first one will be attached to centre of first rod and the second one will be to the end of second rod. Now I bring them ...
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8answers
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What is a rocket engine thrusting against in space?

I know Newton's third law of motion might be the answer for this but still I am wondering how the rockets could thrust in the empty space and move in the opposite direction. I guess an astronaut ...
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1answer
40 views

Complex scalar field

In his book on Quantum Field Theory, Ryder mentioned in p. 91 under the title Complex Scalar Fields and Electromagnetism, the following: He said that under a global phase transformation $$\phi ...
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1answer
51 views

Determine path of point mass using the Hamilton's principle

I am very new in this field but I try to solve a problem by using the Hamilton's principle and afterwards I want to compare the solution by solving the same problem using conservation laws. What I ...
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0answers
17 views

Finding final velocity of neutron [closed]

A neutron moves with velocity $u$. It collides head-on with a stationary nitrogen nucleus. The collision is elastic. The mass of a nitrogen nucleus is 14 times that of a neutron. After collision ...
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1answer
66 views

What would happen if Newton's Cradle was made of other geometrical objects rather than spheres?

What would happen if Newton's Cradle was made of other geometrical objects rather than spheres? For example, what would happen if it was made of cubes and the contact area was larger?
3
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1answer
100 views

How does conservation of energy manifest itself quantum mechanically?

We know that classically, if we have some theory $\mathcal{L}$ such that the action $\int d^4 x \mathcal{L}$ is invariant under time translation, then we can use Noether's theorem to find that (the ...
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7answers
1k views

Does Newton's third law apply to momentum or to forces?

I read all the previous answers concerning the 3rd law and I have seen that it is definitely not universal, (Edit: but conservation of momentum is. If it is not universal it should be not a problem to ...
3
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1answer
69 views

Why does matter gather as discs around massive objects? [duplicate]

Why do discs, like rings of Saturn and the spiral shape of our galaxy form around massive objects, instead of just a (spherical?) cloud of matter?
3
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1answer
139 views

Intuitive meaning of a special case of the Bernoulli equation

In the Bernoulli equation, if $h$ equals zero, it reduces to $$P_1+\frac12\rho v_1^2 = P_2+\frac12\rho v_2^2$$ The equation does not have an intuitive meaning other than the fact that it is a bare ...
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1answer
150 views

In QED, why is the $e^- + e^+\leftrightarrow\gamma$ process forbidden on-shell?

QED has a vertex that couples a single photon to two fermions. This vertex describes the annihilation of an electron-positron pair into a photon. Why is this process forbidden for all three particles ...
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5answers
123 views

Relation between the conservation of Energy and the conservation of Momentum?

I just learn about the momentum and it conservative attribute in a closed and isolate system and there is something I don't understand when I relate it to the conservation of the energy Considering ...
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1answer
109 views

Case of the mysterious bullets (taken from Mad About Physics)

"Two ideal bullets, identical in shape, size and mass, strike the same target with the same speed just before the collision. Force meters at the target register two times the force value for bullet A ...
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2answers
53 views

Effect of movements of astronauts on International Space Station

I know that astronauts move in and around in ISS. When they move they also touch the modules of ISS and sometimes they apply force on the module to move. When this happens, as far as I know it affects ...
2
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1answer
55 views

The massive Thirring model

I am trying to find conservation laws in the following coupled equations: \begin{equation} -af(x) + i\frac{\partial f}{\partial x} + g(x) + |g(x)|^2 f(x) = 0 \end{equation} \begin{equation} -ag(x) - ...
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7answers
2k views

Does electron being many places at the same time violate Physics laws?

The following passage has been extracted from the book Parallel Worlds, by Michio Kaku: Because of uncertainty, the electron does not exist at any single point, but exists in all possible ...
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3answers
979 views

Changing Rotation Direction in Mid Air

Not sure whether this is an appropriate question for this site, but could anyone explain the physics behind how this skier is able to change his direction of rotation mid-air? ...
5
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1answer
86 views

How does the divergence theorem justify the integral interpretation of the continuity equation?

I vaguely understand the continuity equation (at least its integral interpretation), but I don't really understand the differential interpretation of the continuity equation. I'm having trouble ...