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

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If a truck collides with a car, can the truck experience a larger force?

I am confused, here is a question: A large truck and a mini bus both have same velocity V and they collide and stop. The collision lasts for 1 second. A) Which one of the two will experience ...
3
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4answers
3k views

Application of Kirchhoff's laws in circuits with inductors

As we know,the Kirchhoff circuit laws are applicable for conservative electric fields. Now it is applicable for circuits where inductors are present but the field there is not conservative. So how ...
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4answers
255 views

Can conservation of momentum and conservation of energy explain every possible event in the Universe?

I heard my friend, a researcher, say that we can, in theory, explain every event happening in the universe using the Conservation of momentum and energy. He added that we may not be able to do that "...
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3answers
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When is momentum not conserved?

What are some common examples where momentum is not conserved? This question arose in my mind when I read that a ball dropped from a height penetrates into a bed of sand and that momentum is ...
2
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1answer
277 views

$G$-parity in an electromagnetic decay

I am looking at the decay $\eta\rightarrow\pi^+\pi^-\gamma$ and I would assume that the decay itself (ignoring the $\pi\pi$ final state interaction that is obviously strong) is electromagnetic since ...
12
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1answer
893 views

How can a planet gravitationally capture objects?

I would expect that any asteroid or other object originating far away but passing near a planet would pick up speed and energy as it approaches, but unless it comes into contact with the atmosphere (...
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1answer
137 views

Integrals of Motion for s Degrees of Freedom

From Landau & Lifshitz, Classical Mechanics, the number of integrals of independent integrals of motion for a system of $s$ degrees of freedom is $2s-1$. I am considering a spherical pendulum in ...
3
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2answers
440 views

Particle number conservation equals $U(1)$-symmetry?

If have by now frequently read the above but never really understood it. It is said that the particle number conservations is related to the phase of the wave function, but how?
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2answers
1k views

A spring with two masses in an inelastic collision

An ideal spring is attached to a wall, and the other end is attached to a mass $m$. The spring is initially compressed a distance $x$. After it is released, the mass collides with another mass $2m$ ...
2
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2answers
399 views

Conservation laws and continuity equations

I'm a bit messed up with conservation laws and continuity equations. This is my understanding: A conservation law describes that a physical quantity $G$ is conserved with time. It does not prevent ...
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1answer
233 views

Relativistic Conservation laws [closed]

Conservation of Relativistic mass and thus energy is easily proven by considering an inelastic collision of two bodies while invoking the conservation of momentum. As such the momentum law appears ...
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4answers
1k views

Neutron to antiproton decay

Would it be possible for a neutron to lose a positron and become an antiproton? Or would would it need to be the decay of an antineutron to antiproton instead?
3
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4answers
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Conservation of momentum but not kinetic energy in inelastic collisions

In inelastic collisions, the kinetic energy of the system is not conserved but the momentum is. Kinetic energy is: $0.5 \times \text{mass} \times \text{velocity}^2$. Momentum is: $\text{mass}\times\...
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2answers
902 views

Will I be able to push a small object in front of me in the outer space?

Imagine I am standing on Earth, and pushing a tennis ball away from me. The ball moves. If it is very heavy, I will move back instead of the ball. Now consider the same scenario in outer space, where ...
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3answers
2k views

Application of Newton's third law

By newton's third law , we know that every action has an equal and opposite reaction .We move left when we push slightly on the right and move forward as we push backward. But how do rockets, ...
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3answers
19k views

Why is momentum conserved in an inelastic collision and kinetic energy is not conserved? [duplicate]

We know that in an inelastic collision that total momentum of the system before collision equals the total momentum after collision. But total kinetic energy before collision is not equal to total ...
3
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3answers
1k views

Conservation of momentum and energy in an explosion

One simple problem is physics is to determine the mechanical energy difference after an explosion. To do this, you must assume that momentum is conserved because in a explosion you have internal ...
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2answers
199 views

Why isn't jumping against a wall an elastic collision?

According to this calculator http://www.abecedarical.com/javascript/script_collision1d.html when low mass object hits high mass object it is reflected gaining opposite velocity almost the same as ...
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4answers
2k views

Can momentum be conserved in a perfectly elastic collision?

I am taking for granted that when we say that something is conserved it is understood 'in its full integrity'. Energy is represented by a scalar J, and is conserved in elastic collision. momentum ...
3
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1answer
75 views

Relationship of multiple particles under collision [closed]

Consider 3 particles. All 3 particles travel along the x-axis. The 1st particle possesses some mass, m, and its initial position is somewhere on the negative x-axis. It has some (positive) velocity ...
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0answers
35 views

Relationship of multiple particles under collision [closed]

Consider 3 particles. All 3 particles travel along the x-axis. The 1st particle possesses some mass, m, and its initial position is somewhere on the negative x-axis. It contains some (positive) ...
3
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1answer
431 views

Conservation of linear momentum, when is it conserved?

Will Linear momentum be conserved in a non-inertial frame of reference? In other words what is the fundamental condition for linear momentum to be conserved? Also which is more fundamental- Newton's ...
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2answers
157 views

The charm of the gyroscope [closed]

I am fascinated by the gyroscope, like everyboy who was so lucky as to get such a toy as a kid. But probably also grown-ups are not immune to its charm. I am not asking for a theoretical explanation. ...
2
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1answer
540 views

Angular momentum, what is it, is it conserved, and how do we know?

Firstly, most definitions of angular momentum assume a point about which you define angular momentum. I realize that you can consider the angular momentum about any point, and have many angular ...
3
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1answer
94 views

Angular momentum in an accretion disk

I need to plot the time evolution of the total angular momentum in an accretion disc. This confuses me because I thought this should be constant, since angular momentum has to be conserved? I'm given ...
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0answers
78 views

Conserved charge from conserved current associated with translational invariance

(c.f Di Francesco, 'Conformal Field Theory' P.45) Di Francesco calls the conserved charge arising from the conserved current associated with a translation invariant theory the 'four momentum'. While ...
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1answer
77 views

Angular Momentum Conservation Definition [duplicate]

Did I missed something in angular momentum definition? Two identical bodies rotate around mass center. Now I invented anti-gravity and turning gravitational switch off. Those two bodies will move now ...
0
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2answers
151 views

Conservation in space-time curvature

Pardon this possibly naive question. I'm starting to poke around in the topic of General Relativity (as soon as I can pull myself back up out of the vortex of underlying mathematics that I've gotten ...
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1answer
126 views

When I move my arm forward in vacuum, will my body move backward?

Let's say I stay at point $x=0$ in vacuum. When I move my arm forward such that it will have a positive $x$ position (say $x=5$) will the rest of my body move backward such that it will have a ...
4
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1answer
434 views

Crystal Momentum in a Periodic Potential

I'm working through some basic theory on periodic potentials, and I would appreciate help in understanding the crystal momentum. Suppose we have a Bravais lattice with lattice vectors $\textbf{R}$. ...
2
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1answer
68 views

Deriving conserved currents by promoting parameter

I currently reading Tong's text on String Theory. In Chapter 4.1.1 he alludes to a technique to derive conserved currents Recall that we can usually derive conserved currents by promoting the ...
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2answers
325 views

If a ball spinning on a rod hits another ball, what is conserved linear or angular momentum?

Suppose a 1-kg ball A is fixed to a spoke 0.2 m long, which is attached to an axle so that the ball can rotate (v=10m/s, KE=50J, $\omega$=50 rps, L=2, p=0) Now, there is a second ball B (m=1kg), ...
0
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1answer
135 views

Conserved current for a constant translation of a free massless scalar field

In Zinn-Justin's Quantum Field Theory and Critical Phenomena they start with an action for a free massless scalar field: $$S(\varphi) = \frac{1}{2}\int d^{2}x\left[\partial_{\mu}\varphi(x)\right]^{2}$...
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3answers
438 views

Does the the quantum field theoretic process of particle–antiparticle annihilation break the axioms of Special Relativity?

$\textbf{Note that this diagram hasn't anything to do with the question directly.}$ After a particle and its antiparticle annihilate, their energy is converted into a force carrier particle, such as ...
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1answer
153 views

What's the point of NASA's impossible space engine? Wouldn't a mirror work better?

My understanding (which is very little) is that the point of NASA's recent virtual-particle engine is to convert solar energy into momentum. That's fine, but what's the point? If the spacecraft is ...
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NASA's “Impossible” Space Engine

Recently, there was some news that said that the researchers at NASA have come across some impossible kind of space engine which does not require any fuel. I have read at a few places like here, here ...
2
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1answer
267 views

Finding direction of a ball after collision in cartesian coordinate system [closed]

In elastic collision of ball to wall along x axis m*Vix=m*Vfx as velocity of wall is 0 before and after collision thus Vix=Vfx ......eq(1) Kinetic Energy is conserved so m*Vi2 = m*Vf2 (Vix2 + Viy2)=...
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How obtain conserved quantities in integrable models in accordance with Liouville's theorem, via Sklyanin Poisson algebra?

In classical integrable models, in the discrete case we have the Sklyanin algebra, $$\lbrace T_{a}(u),T_{b}(v)\rbrace =[r_{ab}(u,v),T_{a}(u)T_{b}(v)].$$ How to prove that the conserved quantities are ...
5
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1answer
188 views

What's the corresponding symmetry of enstrophy conservation?

In fluid mechanics, especially 2D turbulence study, people talk about conservation of enstrophy. But I can't really understand enstrophy very well, and what's the corresponding symmetry of enstrophy ...
2
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1answer
2k views

Please explain the flaw in this picture [duplicate]

So I saw this picture on my google+ feed and I immediately know why it wont work. But I'm having trouble explaining to myself and others exactly why. Without using anything overly complex, can anyone ...
8
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3answers
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What determines whether a pool ball will bouce backwards after colliding with another pool ball?

I'm no knowledgeable pool player, but I've noticed that sometimes when the cue ball hits another pool ball, they roll together; and sometimes the cue ball bounces back. And I have a very, very rough ...
3
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4answers
2k views

How to calculate a collision which is partly elastic and partly inelastic?

(For the purpose of this question, "calculating a collision" means: given the velocities and masses of two objects in a collision, figuring out the new velocities of both objects after the collision). ...
3
votes
1answer
1k views

Hamiltonian Noether's theorem in classical mechanics [duplicate]

How does one think about, and apply, Noether's theorem in the classical mechanical Hamiltonian formalism? From the Lagrangian perspective, Noether's theorem (in 1-D) states that the quantity $$\sum_{...
1
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1answer
278 views

Conservation of energy and momentum via the continuity equation in asymmetric time and space translation

I am confused about energy and momentum conservation, time and space translation symmetry, and the continuity equation. Suppose we have a mass $m$ in inertial space far from any gravitational ...
3
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4answers
695 views

Is the principle of Conservation of Energy empirically verifiable?

Before I am inundated by myriad and vociferous claims that conservation of energy is the single most well-attested and experimentally verified principle in all of science, let me say that I am well ...
3
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2answers
212 views

Color-charge conservation in proton decay

In some extensions of the Standard Model of particle physics (Supersymmetry with R-parity violation being a prominent example), the proton is allowed to decay, e.g. via $p\to e^+\pi^0$: While this ...
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2answers
2k views

Why is scattering vector $\vec{q}$ called vector of 'momentum transfer'?

In the world of scattering the angle at which you detect the scattered radiation is known as $q$, where $$ \vec{q} = \frac{4\pi\eta}{\lambda}\sin(\theta/2) $$ I read in a lot of books that this is ...
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1answer
118 views

Reversing Noether's theorem [duplicate]

Noether's theorem states: any differentiable symmetry of the action of a physical system has a corresponding conservation law. Is this statement invertible? I mean, if a conservation law exists, this ...
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3answers
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Conservation of 4-momentum in special relativity

I understand that the inner product of two 4-vectors is conserved under the Lorentz transformations, so that the absolute value of the four momentum is the same in any reference frame. This is what I (...
3
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1answer
165 views

What's the conserved quantities correspond to the generator of conformal transformation

What's the conserved quantity corresponding to the generator of conformal transformations?