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

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2
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1answer
72 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 ...
1
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1answer
235 views

QED: basic process which violates energy-momentum conservation

Why is the following process not possible? My book says it's because it violate energy and momentum conservation. Can someone explain me explicitly why? Why couldn't an energetic electron not ...
2
votes
1answer
218 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 ...
0
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3answers
128 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 ...
0
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1answer
63 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?
3
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3answers
2k views

Newton's 3rd Law: How can I break things?

If I punch a wooden board hard enough and it breaks in two, has the board still exerted a force of equal magnitude on my fist? When the board breaks in two due to my force, the halves have a ...
8
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7answers
3k 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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2answers
73 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?
-1
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1answer
189 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 ...
12
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1answer
818 views

Neutrino Oscillations and Conservation of Momentum

I would like to better understand how neutrino oscillations are consistent with conservation of momentum because I'm encountering some conceptual difficulties when thinking about it. I do have a ...
30
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5answers
3k views

Is the converse of Noether's first theorem true: Every conservation law has a symmetry?

Noether's (first) theorem states that any differentiable symmetry of the action of a physical system has a corresponding conservation law. Is the converse true: Any conservation law of a physical ...
3
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4answers
334 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 ...
1
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2answers
59 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?
8
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4answers
15k views

How does the 'water jet pack' work?

So I was cruising around at YouTube and saw this sweet video, and as I was watching started to wonder: "How is this possible?". For a little bit of background, in case you decide to not watch the ...
3
votes
1answer
72 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} = ...
2
votes
7answers
995 views

Why is momentum conserved (or rather what makes an object carry on moving infinitely)?

I know this is an incredibly simple question, but I am trying to find a very simple explanation to this other than the simple logic that energy is conserved when two items impact and bounce off each ...
4
votes
4answers
209 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 ...
2
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2answers
321 views

Law of conservation of matter

If scientist have made small particles of matter then why do we still haw the law of conservation of matter? Is it because the few particles don't make a noticeable difference in our life?
1
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1answer
98 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 ...
0
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0answers
42 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 ...
0
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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
votes
0answers
45 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 ...
2
votes
2answers
188 views

How many hours will be in a day if the radius of Earth increases by 70 m? [closed]

I am little confused about the linear momentum and angular momentum, will the linear momentum of earth change due to changing of its radius or it will stay as it was and i know that the moment of ...
1
vote
1answer
150 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 ...
0
votes
1answer
73 views

Momentum conservation in a small time interval

I recently solved a problem in which I used the "fact" that the momentum is conserved just before and just after a collision between two (or maybe more) objects but I am not sure whether this is true ...
1
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1answer
68 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 ...
6
votes
1answer
80 views

Is there a systematic way to obtain all conserved quantities of a system?

I'd like to know whether, given a system, there's a way to obtain all the conserved quantities. For instance if the system consists of electric and magnetic fields, the fields must satisfy Maxwell's ...
3
votes
1answer
116 views

Charge not conserved in scalar QED? [duplicate]

Since conservation of charge seems to be a well known concept, I am hoping that I am missing something and that the conclusion is incorrect. However, I have been unable to disprove this. Let me ...
0
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2answers
112 views

Conservation of momentum - finding the angle between broken parts

Let a stationary block explodes into 3 equal pieces .The speeds are also equal which is $v$. If I am asked to determine the angle between the broken pieces that are moving then how to deal with this ...
3
votes
1answer
89 views

If a photon doesn't necessarily travel in a straight line, doesn't it defy the law of cons. of momentum?

I just finished reading Richard Feynman's lectures on Quantum Electrodynamics (QED: The Strange Theory of Light and Matter) and it fascinated me. However, there's an unanswered question I have from ...
3
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2answers
200 views

Light and momentum question? [duplicate]

Each photon of light bulb carries momentum. Why does the light bulb not recoil from conservation of momentum?
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2answers
853 views

Why does a Ball bounce back if Forces are Equal and Opposite?

When a ball is thrown onto a wall, the ball exerts a force onto the wall. According to Newton's Third Law, the wall will exert an equal and opposite force to the ball. Thus, how would the ball be able ...
0
votes
1answer
55 views

What will happen if all the population all over the world jumped together at the same time in sea? [duplicate]

What can happen if a magnitude of such huge force is applied over the sea water of earth? Force applied will be average mass of each person *7 billion *9.8 .
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2answers
178 views

Thrust in space without expelling mass

The classic rockets move losing their mass in one direction and moving to the other direction. Thinking about an alternative engine where we lose very small mass. This is how the engine works The ...
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 ...
1
vote
7answers
426 views

If we attach a fan at the end of a boat and switch it on, will the boat move? [duplicate]

I've been across a lot of questions like these, but I'm never quite able to get them. In this question, I reasoned that if I consider the boat and the fan to be a system, then there is actually no ...
1
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0answers
44 views

Noether Charge and Gauge Fields

I understand the global gauge symmetry results conserved charges associated with the symmetry. My question is, why don't we have conserved charges associated with local gauge symmetry of the gauge ...
2
votes
0answers
28 views

Lens-Mirror systems and conservation of specific intensity

This came out of a discussion I started yesterday and a related discussion I found. I'll recap the problem quickly: Consider two blackbodies, with surface areas $A_1$ and $A_2$ and temperatures $T_1$ ...
1
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2answers
43 views

Collision of Inelastic ball above the ground

An inelastic ball of mass $m$ is dropped from a height $h$ above the ground and at the same time a second ball of mass $m_1$ projected vertically upwards to meet the former. Show that in order that ...
0
votes
3answers
82 views

Are there an even number of particles in the universe

I generally understand that every particle has an anti-particle that is created along with that type of particle with opposite effects (unless there is no charge), and that if that anti-particle comes ...
0
votes
1answer
138 views

Problem conserving 4-momentum at CoM frame in an inelastic collision [closed]

I am confused about the case where mass is not conserved in a collision (not due to relativistic factors). The center of momentum (CoM) frame is not the same before and after the collision. Let's ...
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votes
2answers
125 views

How did physicists arrive at conclusion that the product of mass and velocity is equal to momentum?

How did physicists arrive at conclusion that the product of mass and velocity is equal to momentum? What is the intuition behind $p=mv$? I had trouble finding any sources that state the actually ...
-1
votes
1answer
102 views

work and energy dilemma in an inclined [closed]

An example problem done in my book: A driver was driving a car of mass 1000 kg through an inclined plane which makes an angle of 30° with plane with a velocity of 25 m/s . Then he saw a boy 50 m ...
0
votes
1answer
122 views

Why is the tilt of Earth in sync with an orbit around the Sun?

Is there any physical reason the changing tilt of Earth (which gives rise to our seasons: Spring, Summer, Autumn and Winter) is in sync with an orbit of the Sun (i.e. one year)? It can't just be ...
0
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1answer
34 views

Advent of our solar system

When the solar system was born why did Sun not consume all the planets by its immense gravity, as the planets were all gases, so it would have been very easy?
1
vote
1answer
74 views

Rocket velocity during fuel burn [closed]

I know the formula for the velocity of a rocket after the on board fuel has been used up: $$ \Delta v = v_{ex} \ln\left( \frac{m_i}{m_e} \right) - gt $$ where: $v_{ex}$ is the exhaust velocity ...
1
vote
1answer
1k views

How did one get the defining equation of probability current and conservation of probability current and density?

I'm reading the Wikipedia page for the Dirac equation: $$\rho=\phi^*\phi$$ and this density is convected according to the probability current vector $$J = ...
0
votes
1answer
53 views

Can high energy electrons slow down by emitting photons?

When a charged particle is moving with large kinetic energy in the Earth's reference frame, can it emit a photon effectively slowing it down in this frame?