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

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32
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Intuition as to why the orientation (of a 3D object) is not a conserved quantity?

Say you start off floating in space, in a fixed position and orientation, with zero linear and angular velocity, with no external forces. So you are a closed mechanical system. By twisting your body ...
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1answer
311 views
0
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1answer
124 views

Basic question about angular momentum

I've learned that the angular momentum of an object rotating about a fixed axis is $I \omega $. Also, in absence of external torques, $I_1 \omega_1 = I_2 \omega_2 $ (meaning, two different events). I ...
0
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0answers
31 views

Does the similarity of gamma matrices correspond to a conserved quantity?

Gamma matrices have a similarity property, $\gamma^\mu\to S\gamma^\mu S^{-1}$ is a good transformation. Does this transformation correspond to a symmetry of the QED Lagrangian?
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0answers
52 views

Accelerating rockets by periodic explosions? [closed]

Since in the outer space friction is almost 0, couldn't we accelerate a rocket to reach much faster speeds than that of the current limit by periodically starting and stopping the engines? I think ...
0
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2answers
100 views

How does a wheel balance itself during circular motion? [duplicate]

A wheel (or any ring of considerable mass) hardly balances itself when it is placed vertically on ground, but when we roll it along the ground it balances itself. What causes this effect? I guess its ...
0
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1answer
29 views

Ballistic pendulum - are all forces conservative for this case?

For the Ballistic pendulum in the image below: Are we allowed to assume that after the bullet hit the log (mass M), then there's a conservation of energy? (Thus $\frac{1}{2}(M+m)V^2 = (M+m)gh$) ...
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1answer
204 views

Elastic Collisions and Relative Velocities

In a 1D elastic collision, it is well-known that the relative velocities of the two objects (before and after the collision) are reversed. What is the extension of this result to 2D or higher? Is ...
2
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2answers
166 views

Is momentum conserved in the collision of a ball with a hanging rod?

Suppose we have a situation like A ball of some mass $m$ with some velocity collides with rod hinged at point $A$. Is momentum conserved in this situation? I know that hinge will give impulsive ...
2
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1answer
81 views

“Where” does dissipated enstrophy go?

We are all familiar with the kinetic energy dissipation and how it is converted into heat which can either be radiated away or go into the internal energy of the system. In the enstrophy transport ...
2
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1answer
80 views

Energy and momentum conservation - why it is so fundamental?

Over hundreds of years the conservation of energy and momentum in a closed System was proven. 100 years ago, Emmy Noether showed that these fundamental laws arise from the following facts and vice ...
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1answer
51 views

Relationship between energy density and energy flux

I'm presently working on obtaining conservation laws via symmetries. These conservation laws are written as 2-element vectors where each element is the energy density and energy flux. To proceed in my ...
2
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0answers
56 views

Off-shell legs in Feynman diagrams

I have a tree-level diagram with one leg being off-shell (its momentum beeing $\mathcal{O}(m_B)$). How do I treat this leg when computing the amplitude? Do I put in the propagator and ignore the ...
0
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1answer
42 views

Rolling on a frictionless pond

I have a doubt about one of the questions in my textbook. Q- You are standing with your bag in your hands in the middle of a friction-less pond. How can you come out of the ice? There is of ...
2
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1answer
43 views

Separability of Hamilton Jacobi Equation

When we talk about integrability of classical systems in terms of Hamiltonian or Lagrangian mechanics, it's all to do with counting independent conserved quantities. Then when we move to the ...
0
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1answer
130 views

Why do planets not stop revolving around the Sun? [duplicate]

Why do planets revolving around the Sun not stop revolving? Note I am not asking why planets do not collapse with Sun.
0
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1answer
27 views

When is it appropriate to drop pressure terms when applying conservation of momentum to a fluid?

I'm trying to wrap my head around pressure forces in incompressible, irrotational, invicid flow. Applying conservation of momentum to a control volume gives me \begin{equation} ...
0
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2answers
382 views

Proof that the electric field is conservative

I was told a proof that the electric field was conservative (without using $\nabla$) which used a point charge and showed the following: $$w.d.=\int_c{\vec F \cdot \mathrm{d} \vec l}=\int_c{\vec ...
5
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4answers
211 views

Thermodynamics thought experiment

There is some ideal gas in a container moving with some velocity on a smooth surface and you suddenly stop it( say by using your hands) , will the temperature of the gas increase? It seems to me that ...
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2answers
168 views

How to use the first law of thermodynamics for simple mechanical systems?

I'm confused about what exactly is $Q$ and $U$ and their signs. Consider a block initially having some kinetic energy which we stop and we want to find by how much amount its temperature increases. ...
5
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2answers
262 views

Is it possible to determine the outcome of any impact knowing only the ratio of masses? [duplicate]

In elastic collisions in 2-D if two balls $A$, $B$ ($m_A = m_B$, $R = 1$) have equal mass we can determine in advance the outcome of the collision. If cue-ball $A$ impacts object-ball $B$ (at rest) ...
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2answers
125 views

Energy conservation in electrodynamic system?

Consider two charged particles initially at rest in the configuration below. Let us assume the following: Starting at time $t=0$, we apply a constant force $f$ to the the bottom particle so that ...
6
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3answers
180 views

Is it in general true that $\nabla_\mu T^{\mu\nu}=0$ implies the matter equations of motion?

I know of several cases where the covariant conservation of the energy momentum tensor $\nabla_\mu T^{\mu\nu}=0$ can be used to derive the equations of motion of the matter fields. Is this in general ...
0
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1answer
206 views

Conservation of angular momentum in a free rod

When a collision is elastic and no external torque acts on a system, angular momentum is conserved I found this example and checked the results: A ball (m = 1 Kg , v = p =+22 m/s, Lm = +11, Ke = 242 ...
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2answers
148 views

Classical EM : clear link between gauge symmetry and charge conservation

In the case of classical field theory, Noether's theorem ensures that for a given action $$S=\int \mathrm{d}^dx\,\mathcal{L}(\phi_\mu,\partial_\nu\phi_\mu,x^i)$$ that stays invariant under the ...
2
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1answer
51 views

How to derive the conserved current of the Klein Gordon equation?

Similarly to the probability current in non-relativistic quantum mechanics, there is a conserved current for the Klein Gordon equation, however a different one. I'm trying to calculate that. The KG ...
0
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1answer
114 views

How does the space shuttle move in a vacuum? [duplicate]

If there is no friction in space, then how does the space shuttle boost? According to Newton's law we move by pushing something; in space, there is vacuum, so how the space shuttle moves without ...
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5answers
130 views

Physical interpretation of the statement $\oint E\cdot dl=0$

Can anyone provide me with a physical interpretation of $\oint E\cdot d\ell=0$ in electrostatics?
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4answers
142 views

Which is the right explanation for rocket motion? [duplicate]

What actually causes a rocket to move? Is it the pressure in the rocket engine or the amount and velocity of mass that is being ejected out. The reason I am asking is, I found these two explanations ...
0
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1answer
56 views

Reaction in Particle Physics

I'm currently checking some reactions if they are allowed. One of them is $$\nu_\mu p\to\mu^-\Delta^{++}$$ Since this is a weak interaction I've checked the Baryon and Lepton number, Spin and the ...
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2answers
303 views

Photon energies after neutral pion decay

I am trying to find the photon energies of the decay $\pi_0 \rightarrow \gamma\gamma$ and their dependence on the pion energy $E_{\pi}$, its initial velocity $\beta$ and the scattering angle between ...
2
votes
2answers
79 views

Is It Possible To Move The Earth In A Direction? [duplicate]

According to Newton's 3rd law, when an object moves upward, the Earth moves by a very tiny factor in the opposite of the direction of the object. And when the object falls back, the earth moves back ...
3
votes
2answers
53 views

Can momentum be distributed to multiple objects which travel in different vectors?

This is a question about the conservation of momentum: If there were a cluster of billiard balls floating in space and the cluster was struck by one moving ball, the cluster balls would scatter in ...
3
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1answer
134 views

Number conservation of bosons and fermions

Why is the number of bosons not conserved while the number of fermions is conserved? Does it have something to do with the Pauli exclusion principle?
3
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1answer
156 views

Paradox: electric current in a coil on a disc - will this disc spin if the circuit is opened?

I encountered this problem in a book, but there were no solution written there. The setup: there is a plastic (insulator) circular disc, that is suspended in a way, that can very easily rotate (so ...
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2answers
191 views

Question about impulse and momentum

Check this question first please My question is: Why can't I use equations of motion to get the final speed after rebounding?Acceleration not equal to $9.8 \text{m/s}^2$ or the intial after ...
0
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0answers
171 views

Experiment to verify a spring constant

In my high school physics class, I had students do a Hooke's Law lab to measure an unknown spring constant. Once we compared everyone's data we came up with a class value for $k$ of 253 N/m. We then ...
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1answer
58 views

What is the ultimate “goal” of two particles in a gravitational field? [closed]

What is the ultimate "goal" of two particles in a gravitational field? Will the particles eventually occupy the same point at the same time if nothing stops them? What stops them from achieving the ...
0
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1answer
68 views

Scalar field gives spin $0$ particles

Related to this question. What needs to be shown is that the total angular momentum (derived from the Klein-Gordon lagrangian) acting on a ket with $0$ momentum, gives 0. This would mean that a ...
1
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1answer
67 views

What is going on in a rotating magnet in a quantum scale?

If there is a rotating magnet in an empty space and there is no outer field acting on it. Rotating in such a way that after half rotation magnet's N pole will be in the place where magnet's S pole ...
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0answers
102 views

Law of Conservation of Energy

I find it hard to comprehend the law of conservation of energy. Allow me to explain my confusion. I understand that the law of conservation of energy states that energy is neither created nor ...
2
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1answer
67 views

Diffracted electron - where does it gain additional momentum?

When an electron is diffracted, the momentum after the diffraction has different direction than before. Where does the electron gain this momentum? This is related to this question, but it's ...
0
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0answers
114 views

Spin, parity, etc. conservation in decays/reactions

I have a long list of short physics questions I'm reviewing, and there are several that ask about whether certain decays or reactions are allowed. Clearly these questions are testing my knowledge of ...
3
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1answer
105 views

Trying to show that the current is conserved

$ \newcommand{\p}{\partial} $ I am trying to show that the current $J^{\mu} = (\gamma_{\nu}\partial^{\nu} \phi - m\phi)\gamma^{\mu}\psi$ is conserved for all fields that satisfy the Klein-Gordon and ...
2
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1answer
76 views

Why is the Earth self-rotating? [duplicate]

What drives this happen? Would it be the internal energy or by an external force? I did try to Google the answer, but could not find a good one.
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0answers
63 views

Determining Parity of Decaying Quantum System

Show that a particle of spin $1$ cannot decay into two identical particles of spin $0$. The $\rho$-meson has spin $1$ and can decay into two spinless (spin-$0$) $\pi$-mesons, or pions, with ...
3
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5answers
411 views

The elusive difference between force and impulse

Impulse is defined as the product of a force $F$ acting for a (short) time $t$, $J = F*t$, and that is very clear. What I find difficult to understand is how a force can exist that doesn't act for a ...
5
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2answers
150 views

Why is it important that the vector current should be conserved in QED?

In Quantum Field Theory and the Standard Model by MD Schwartz in the chapter about the anomalies, he derives from the equation of motions and the Noether currents of a effective massless QED ...
0
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1answer
29 views

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
200 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 ...