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

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62 views

First law of thermodynamics with additional term

I read in a paper that a "known expression for the heat received by a body" is $$dQ=dU+pdV-\mathbf{v}\cdot d\mathbf{P}$$ where $\mathbf{P}$ is the linear momentum of the body, $p$ is the pressure, $U$...
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3answers
144 views

General Definition of Steady State

According to many sources (including Wikipedia, Stephani&Kluge, D.J. Acheson) a steady state ist: In systems theory, a system in a steady state has numerous properties that are unchanging in ...
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2answers
180 views

Is the reaction force for a stone hitting a wall infinite?

Let us assume a rigid stone which moves in empty space with a constant speed of $v$. (Or in the air with no friction and drag or you can imagine a free fall with friction). This stone hits a rigid ...
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0answers
17 views

Motion in Spaces [duplicate]

How does a shuttle move in the space as, the whole mechanism of a rocket is based on 3rd law of newton but, we can not apply 3rd law of newton in space because there is no reactive force in space?
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1answer
134 views

Explain this contradiction of violation of “ energy conservation ” using classical mechanics

Consider a rocket moves upward with some acceleration for a very small time say '$dt$' then the kinetic energy increases (for acceleration). as well as as the potential energy increases (due to height ...
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0answers
67 views

Dirac equation in the algebra of physical space and conservation laws

I have the following question: I was thinking, is it possible to obtain the conservation laws for the Dirac equation in the algebra of physical space? If yes, how? Can anyone show me a book for these ...
2
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1answer
98 views

When are energy, mechanical energy, momentum, and angular momentum conserved?

I am in AP Physics and my only real hangup is knowing when the said quantities are conserved. Please define what is the SYSTEM in your answer. I kind of have the basic idea. For example, if there ...
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4answers
512 views

Gravity between two unequal masses. Do both masses move?

I've been watching videos about gravity and I have a question My understanding is that mass have gravity and gravity is a force which attract other object with mass. For example, I jump up and the ...
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3answers
97 views

Can a neutron decay to the gravitons?

Is it possible that a bunch of neutrons totally decay to the graviton? In other words, does the baryon number conserve in the quantum gravity interactions?
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1answer
54 views

Applying conservation of energy to a railgun problem?

When a projectile is lunched due to the Lorentz force($F_L$), how can I apply the conservation of energy that the electrical energy inputted to generate the Lorentz force & magnetic field equals ...
2
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1answer
55 views

Simple frame of reference problem (conservation of momentum?)

I'm having trouble wrapping my head around a particular concept. Suppose we have a machine that fires balls of speed $u$ at some mass rate $\sigma$ (of units $\frac{kg}{s}$) directly at a car of mass $...
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2answers
216 views

Conservation of energy and Killing-field

In general relativity we have no general conservation of energy and momentum. But if there exists a Killing-field we can show that this leads to a symmetry in spacetime and so to a conserved quantity. ...
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1answer
40 views

Conservation of momentum in inelastic conservation

Why does momentum and velocity change along contact normal but not contact tangent In in elastic collision? Suppose a ball strike an inelastic surface. It's velocity component does not change along ...
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1answer
34 views

momentum conservation in particle-antiparticle creation

It's understood that the PRESENCE of a heavy nucleus is necessary for conservation of linear momentum in pair creation. What I can't understand is why it must occur in ADJACENCY of the nucleus. Is ...
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2answers
209 views

Is there something more to Noether's theorem?

From the definition of Lagrangian mechanics, Noether's theorem shows that conservation of momentum and energy comes from invariance vs time and space. Is the reverse true? Are Lagrangian mechanics ...
2
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1answer
79 views

Does third law of motion apply to light or EM waves? [duplicate]

Third law of motion - "For every action there is an equal and opposite reaction" I was considering the situation, where I may be motionless in space with only a flashlight and no forces acting on me. ...
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1answer
31 views

The equation of continuity in isothermal system in spherical axis(transport phenomena)

My homework is about finding the equation of continuity in isothermal systems in spherical axis, I can't imagine a workaround for that since its a little complicated for me to understand velocities ...
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1answer
74 views

Conservation of momentum homework problem [closed]

A 2-stage rocket travels $1200 \text{ m/s}$ relative to earth. When the first stage runs out of fuel, the explosive bolt separates the first stage from the second with a velocity of $35 \text{ m/s}$ ...
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1answer
131 views

Difference between $dM/dt = 0$ and $\partial M/\partial t=0$ [duplicate]

$\frac{dM}{dt} = 0$ represents a constant of motion $M.$ Why not $\frac{\partial M}{\partial t}$ represent a constant of motion $M$?
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5answers
886 views

Why can't I push myself in a chair? [duplicate]

If I am sitting in a chair with wheels and someone pushes on the back of my chair with sufficient force it will role along the ground. However, if I push on the back of the chair with the same force ...
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2answers
170 views

Particle anti-particle annihilation and photon production

This is just a conceptual question I guess. The annihilation of a particle with a finite mass and its anti-particle cannot lead to the emission of only one photon, and this is due to the conservation ...
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1answer
58 views

Conservation of angular momentum poisson brackets vs newtonian mechanics [closed]

So I have the following system. For a mass falling due to gravity the given Hamiltonian is $$ H = \frac{1}{2m}\left( P^{2}_{x} + P^{2}_{y} \right) + mgy $$ In Cartesian coordinates then, $$ x = v_o ...
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0answers
44 views

Chiral anomaly and fermion number conservation

Chiral anomalies in QED and QCD violate fermion number conservation, since a U(1) vector symmetry corresponds to fermion number conservation. However, only the LH and RH fermion numbers are not ...
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1answer
395 views

What is the initial velocity of an object, given a change in mass, and an exerted force [closed]

An 8.0-g bullet is shot into a 4.0-kg block, at rest on a frictionless horizontal surface (see the figure). The bullet remains lodged in the block. The block moves into an ideal massless spring and ...
2
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1answer
146 views

Feynman diagrams and gluon collisions/interactions?

We have been given this question which essentially asks us to draw the lowest order Feynman diagrams for various processes. One of them is: $$ g + g \rightarrow \bar{t}+t $$ Now, I am not an expert ...
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2answers
75 views

Heat produced in collision [closed]

A 5-gram marble is moving at 5 m/s [N] collides with a 2-gram marble moving at 3 m/s [N]. Final velocity of 2-gram marble: 7 m/s [N] How would I find the amount of heat produced in this collision? ...
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1answer
204 views

Why is only angular momentum conserved for a planet and not linear momentum?

Suppose a planet is moving in an elliptical orbit with the Sun at one of its focii. I know that the forces in existence will be gravity which provides the necessary centripetal force. Now my book ...
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1answer
77 views

Understanding elastic collisions of objects with the same velocities

I am having trouble understanding the following statement from my book: The law of cosines tells us that if the sides of a triangle obey the Pythagorean formula, they must form a right triangle. ...
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1answer
49 views

Time derivative of Noether charge

I understand that the Noether charge can be written as $$ Q= \int d^3 x J^0$$ and the time derivative of the Noether charge is zero $$ \dot Q=0 $$ but how would you explicitly calculate it?
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1answer
129 views

Are all conservative forces a central force?

If a force is a central force and can be written as $\vec{F}(\vec{r})=f{(r)}\hat{r}$ , then it is a conservative force. But is the converse true? I mean, are all conservative forces a central force? ...
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1answer
176 views

On the definition of hyperbolic conservation law

There is a Wikipedia article and an article by A. Bressan that introduce the notion of hyperbolic conservation law. I'm not used to this area, so I have some questions about the definition. I will ...
6
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1answer
257 views

How does one account for the momentum of an absorbed photon?

Suppose I have an atom in its ground state $|g⟩$, and it has an excited state $|e⟩$ sitting at an energy $E_a=\hbar\omega_0$ above it. To want to excite the atom, one generally uses a photon of ...
2
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1answer
76 views

The role of mass in the tablecloth trick

Apologies if this has already been answered, but I have a question about the role of mass in the classic tablecloth trick, where the demonstrator pulls the tablecloth out from underneath a set of ...
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1answer
234 views

Why does the rule that elastic collisions are at 90 degrees in 2 dimensions not apply?

When one object collides with another object of the same mass in a 2D plane, we know that we can derive that the angles that the objects leave the collision at add up to 90 degrees in a perfectly ...
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3answers
145 views

Work done by a constant vector field is 0?

We know that $$\oint \boldsymbol{F}\cdot d\boldsymbol{r}= \iint (\nabla \times \boldsymbol{F})\cdot d\boldsymbol{s}.$$ Now if $\boldsymbol{F}$ is a constant vector, then $\nabla \times \boldsymbol{F}=...
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1answer
66 views

Validity of law of charge conservation

Charging by induction and earthing gives an object a net charge but why does the law of charge conservation still hold in this case?
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2answers
87 views

Invariance and conservation

Why in a collision between particles is the four-momentum conserved within a frame of reference but not invariant between frames of reference?
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1answer
198 views

Is momentum truly conserved? [closed]

Suppose I have a system of a person holding a rock on a frictionless sheet of ice. He throws the rock at an angle above the horizontal to propel himself off the ice. Is the momentum of that system ...
2
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1answer
139 views

Noether's Theorem for Hamiltonians and Lagrangians

Looking around I see one version of Noether's Theorem that creates conserved quantities from symmetries that preserve the Lagrangian (e.g. http://math.ucr.edu/home/baez/noether.html), and another ...
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4answers
358 views

Conservation of momentum in a baseball

Conservation of momentum: A thought experiment. A baseball is placed on top of a baseball holder, the kind used to train young batters. A batter hits the stationary ball perfectly horizontal, sending ...
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2answers
62 views

How can make myself sure that I can apply Conservation of Momentum in a system?

How do I know I can apply Conservation of Momentum in the System ? And What happens if there is a Impulse in the system?
3
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1answer
78 views

Neutrinos and global $U(1)$ symmetry of Weyl fields

My book on QFT says that neutrinos are well described by left-handed Weyl spinor. The classical Lorentz-invariant Lagrangian density for that field is: $$ \mathcal{L} = i\psi^{\dagger}_L\overline\...
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2answers
211 views

Problem regarding bullet penetration [closed]

Which property remains constant when we say that bullets are penetrating the wall? I mean is the resistive force always constant for a given wall (assumed stationary and immovable) irrespective of the ...
4
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4answers
224 views

Translational invariance implying diagonal representation in momentum space

I have just come across something in my reading of Peskin and Schroeder that claims that because a function, in this particular case a two-point correlation function, is translationally invariant, it ...
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1answer
103 views

Help on understanding a concept in Noether's first theorem

Given a Lie group $G$, whose most general transform depends on $\rho$ parameters, under the action of which an integral $I$ is invariant, there are $\rho$ linearly independent combinations of the ...
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0answers
43 views

Show that $M^{\mu\nu}$ describes the angular momentum of the system

Define $M^{\mu\nu}$ = $\int d^3x(x^\mu T^{0 \nu}-x^{\nu}T^{0 \mu})$ describes the angular momentum of the system. I don't want you to solve it but I'm not really sure what kind of criterion it ...
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0answers
18 views

Discrete translational invariance of lattice systems and conserved quantities [duplicate]

Imagine a crystal lattice with discrete translational symmetry. Is there any way to obtain local periodic conserved quantities by taking a derivative (deliberately left abstract)? The discretised ...
2
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0answers
46 views

Have Witten-type TQFT's nonconservation of energy and momentum in interactions?

Witten-type topological quantum field theories are based on cohomology theories. Every observable must lie in a cohomology class. May be $G$ a geometric field. Then every observable expectation value ...
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1answer
163 views

Invariance of law of conservation of angular momentum under a Galilean transformation [closed]

Given a reference frame O' moving at a constant speed $\vec{V}$ in relation to another reference frame O, I want to prove that $\vec{r_{1B}} \times m_1\vec{v_{1B}} + \vec{r_{2B}} \times m_2\vec{v_{...