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

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31 views

How to solve this stationary object collision problem? (Answers must be numerical.) [closed]

If a mass (m) is traveling at a positive velocity and collides elastically with another mass (m*2) that is at rest, what are the ending velocities of both masses? So far, I've gotten that v1f = -1/3(...
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37 views

Period of a Neutron star? [closed]

I need help understanding this concept of rotational motion. I have a problem that uses a star in which the star has a radius initially of 7*(10^7) km and it is treated as a solid sphere(which means ...
0
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0answers
47 views

Trouble following an argument about a rocket problem

This is from pages 32-33 of Physics for Mathematicians. A rocket + fuel system of mass $m(t)$ is moving along at velocity $v(t)$, with its fuel being ejected at the velocity (relative to the rocket) $...
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1answer
41 views

Collisions; Linear Momentum [on hold]

Let's say we got a rough surface with friction and there are 2 blocks, the block of the left has initial velocity and will collide with the block of the right, which is in rest. Consider an elastic ...
0
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0answers
37 views

Current Conservation [on hold]

Why is current conservation important? Specifically, in QFT and String Theory lecture notes and textbooks it's always stated that current conservation is important because it helps maintain quantum ...
0
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1answer
40 views

Homogenity and Isotropicity of space

In school it is given that law of conservation of momentum is a result of homogeneity of space and law of conservation of angular momentum is a result of isotropicity of space but what is isotropicity ...
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2answers
45 views

Does a circularly polarized electromagnetic wave transfer angular momentum from the transmitter to a receiving antennae?

This question is about the rotation of macroscopic objects and looks at the magnetic vector of an electromagnetic wave. As basis for comparison, we consider an induction motor. The stator induces a ...
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1answer
61 views

If matter cannot be destroyed, does that mean that these particles are created somewhere else after breakdown but in the particles original shape?

In the electron collider, after the breakdown into smaller particles, these particles last for a fraction of a second, and then they disappear. My question is: If matter cannot be destroyed, does ...
37
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1answer
2k views

Why was the first discovered neutrino an anti-neutrino?

In the search for neutrinos, Cowan and Reines discovered the electron anti-neutrino and named it as such. Why is the particle they discovered the anti-variety? The reason we call electrons 'electrons'...
2
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0answers
27 views

Where do the intrinsic parities of particles come from?

It is known that some particles have negative intrinsic parity - for example pion $\pi$. I was wondering if this parity can be understood. I read somewhere that parity of quarks is defined to be ...
3
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0answers
47 views

Integrals of motion for a free particle

I'm struggling to understand the argument on p. 13 in Landau and Lifshitz that for a system with $N$ degrees of freedom there must be $2N-1$ integrals of motion. In particular, I can't understand ...
0
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2answers
82 views

If I throw or kick a ball, how much will the Earth move in the opposite direction?

I understand that for momentum to be conserved, if I throw a tennis ball (or kick a football) the Earth must move in the opposite direction to the ball. Obviously this is an infinitesimally small ...
0
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1answer
120 views

Is conservation of momentum and energy valid for non-inertial frames?

Conservation laws of momentum and energy are said to be the most basic principles of physics. Are they also valid for non-inertial frames, and in what way?
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2answers
50 views

Can a system's internal forces change the system's motion?

Let say the motor makes the two rigid body rotate as a same velocity. (they rotate the opposite direction) then the Net force of the motor is 2Fsin(Theta). The acceleration of the motor is 2Fsin(theta)...
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3answers
55 views

Can internal forces bring any change in momentum of the system?

Imagine a rectangular box placed at the tip of an iceberg; the box contains a man who can freely walk/jump/rotate inside the box. The man walks up to one of the corners of the box and starts jumping. ...
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1answer
52 views

Gravity and angular momentum

When a mass moves towards a bigger mass at a constant velocity, they will be attracted to each other such that the smaller mass will revolve around the bigger mass with the same speed as before. ...
1
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1answer
48 views

Energy conservation and time translations

The time translation is given by a finite unitary transformation $$\hat{U_{\tau}}(\hat{H}) = e^{\big(\frac{i}{\hbar}\tau \bar{H}\big)}.$$ Where $$\hat{U_{\tau}}(\hat{H})|\psi(t) \rangle = |\psi(t-\tau)...
12
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3answers
2k views

What are some phenomena that can not be described without the help of Newton's third law of motion? [closed]

What are some phenomena that can not be described without the help of Newton's third law of motion? All the phenomena I can think of can be explained with the help of Newton's first law or second law. ...
0
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1answer
46 views

Lorentz transformation and symmetries of the Lagrangian [duplicate]

Since the Lagrangian of our quantum field theories is covariant under Lorentz transformations I'm asking myself if there is any link to some symmetries (like that we get from gauge transformations ...
1
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0answers
55 views

Meaning of a Dirac delta speed in 1d shallow-water equations

A 1d thin layer of a fluid of constant density in hydrostatic balance can be modeled by shallow-water equations: $$\begin{align*}\partial_tu+\partial_x\left(v+\frac{u^2}{2}\right)&=0,\\ \...
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2answers
81 views

Prove energy conservation using Noether's theorem

I wonder how you prove that energy is conserved under a time translation using Noether's theorem. I've tried myself but without success. What I've come up with so far is that I start by inducing the ...
2
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2answers
93 views

Newton's Cradle: why does it stay symmetric? [duplicate]

How is it that always the same number of balls leave at the other end in Newton's cradle. I understand that the momentum needs to be conserved, but as momentum is defined as p=m*v couldn't you have a ...
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2answers
112 views

Do conservation laws contradict quantum mechanics? [closed]

Take for example the double-slit experiment interpreted in the Copenhagen sense. The particle leaves as an object with mass, yet passes through the slits as a massless wave, only to collapse again as ...
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2answers
60 views

Where does $W$ boson mass come from in neutron decay?

Here's a diagram of neutron decay. Up and down quarks have rest masses of 2-4 MeV. The $W$ boson has a rest mass of 80 GeV. Where has this extra mass come from?
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1answer
39 views

When does $ S\cdot v=\mathrm{cost}$ hold?

In fluids dynamics what are the limitations for the use of the volume flow rate conservation law? $$ S\cdot v=\mathrm{cost}$$ Does it still hold when there are external forces acting on the system, e....
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1answer
68 views

Under what condition is angular momentum conserved in both classical and quantum physics?

Classically, angular momentum is only conserved in a central potential by considering the torque (correct me if I am wrong). In quantum mechanics, it is also true, isn't it? If this is the case, ...
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1answer
58 views

With the descent of Newtonian mechanics is Newton's third law still valid?

Or more specifically, with the standard model, quantum theory and other advances in physics, all those experiments in CERN and other accelerators, was there any occurrence where this law was violated? ...
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2answers
122 views

When is stress-energy tensor defined as variation of action with respect to metric conserved?

In General Relativity Einstein's equation implies that stress-energy tensor on its RHS is conserved (has vanishing divergence), due to the Bianchi identity. Considering variational principles leading ...
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1answer
48 views

Baryon / lepton number violation equivalence in Sakharov conditions

In the Sakharov conditions for matter-antimatter asymmetry the requirement for the baryon number violation is often interchanged with lepton number violation. In the context of the Sakharov conditions,...
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0answers
45 views

Showing classical spin tensor is time independent for free particle

Reading through Weinberg's gravitation book, the following definition is given for the spin tensor (Pauli-Lubanski psuedovector): $$ S_\alpha = \frac{1}{2}\epsilon_{\alpha\beta\gamma\delta} J^{\beta\...
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1answer
41 views

Intuitive picture for Mandelstam $t$

In tree-level electron-positron scattering one has two possible channels corresponding to Mandelstam variables $s$ and $t$. The $s$-channel ist fine, there $\sqrt s$ is just the center of mass energy ...
1
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1answer
68 views

Does Newton's third law remain totally unchanged even in Einstein's theory?

Why Newton's third law remain unchanged still now in relativity theory (as for example that is why we feel weight due to equal but opposite reaction of the Earth's surface)?
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75 views

Throwing masses to gain velocity. A question of efficiency

I was thinking the other day of a system in which you eject mass. And then I wonder whether it was more efficient to throw a big mass or many little. Let me put it in other terms. Suppose you are ...
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1answer
39 views

Why the constancy of an observable w.r.t time depends on whether it commutes with $H$ or not?

I have been reading Modern Quantum Mechanics by J.J.Sakurai. Under the chapter Quantum Dynamics, the author says if an observable $A$ initially commutes with the Hamiltonian operator $H$, then it ...
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3answers
135 views

Why is this particle interaction forbidden?

Why is this interaction forbidden? $$\nu_e+\bar{\nu}_e\rightarrow K^+ + K^-$$ Lepton number is conserved, charge is conserved, baryon number is irrelevant since these are mesons. Energy is conserved ...
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1answer
57 views

How do you determine which way two objects will travel after collision when given their respective masses and speeds?

I was practicing some Mechanics, and I came across a problem that involved an impact between two objects: I understand how to approach the question: Apply the conservation of momentum, and use the ...
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3answers
53 views

Change in linear momentum [closed]

Let A and B be to rigid blocks. A is stationary while B is moving with a velocity $u$. So if B collides with A. (These blocks are on a frictionless surface.) What would happen after B collides with ...
7
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2answers
710 views

Conservation of momentum when one body is fixed

When we fire a gun, it recoils. Both bullet and gun gain in momentum. Before the shot, momentum of both is zero. If we make the gun unmovable somehow, after the shot, what will be the effect on the ...
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3answers
98 views

When a bomb explodes, does it momentum remain same?

If, from an aircraft, a bomb is thrown to an object placed at ground and bomb explodes before it hits the object, i.e if it explodes in the middle of its path, does it momentum remain same? I knew ...
2
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2answers
63 views

Why doesn't precessing a spin cause a measurement?

When an electron is placed in a magnetic field, its spin precesses. Why doesn't this reveal the spin via angular momentum being transferred into the environment? Impractical Thought Experiment ...
0
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4answers
121 views

Why don't two objects move with the same velocity after collision?

I have a problem with understanding the nature of collisions and their outcomes. From my understanding, I come to think that when a mass collides with another, both of them should always have equal ...
3
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2answers
45 views

Momentum conserving delta-function in the transfer matrix of quantum-field-theoretic scattering theory

The $S$-matrix vanishes unless the initial and final states have the same total $4$-momentum, so it is helpful to factor an overall momentum-conserving $\delta$-function: $$\mathcal{T}=(2\pi)^{4}\...
0
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1answer
40 views

How does Modified Newtonian Dynamics (MOND) violate Newton's laws?

I have read some articles on MOND which indicate that a modification of gravity violate Newton's third law, but I don't know how exactly that works. I do know that if Newtonian acceleration is smaller ...
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1answer
59 views

Conservation of Komar Mass

The definition of Komar Mass in GR is associated with one asymptotically flat end. However, a hypersurface may contain more than one end, such as the spacelike Einstein-Rosen bridge in Kruskal ...
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1answer
79 views

Why are there e.m. vacuüm fluctuations?

According to the rules of qft there are virtual photons in the vacuüm. But how can this be if for the production of photons you need an electric charge?
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0answers
14 views

Momentum conservation problem, elastic collision [duplicate]

This question has got me thinking for a while now and I haven't been able to come up with any answer at all. I've done the basic conservation of momentum, kinetic energy but I have no idea how to ...
1
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1answer
62 views

What is the speed of the skaters in this case? [closed]

You're choreographing your school's annual ice show. You call for eight 60kg skaters to join hands and skate side by side in a line extending 12m. The skater at one end is to stop abruptly, so the ...