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

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2answers
870 views

Angular momentum conservation in pion decay?

I have seen the charged pion decay $$\pi^{-}~\to~ \bar{\nu}_{\ell} +\ell^{-}$$ represented with diagrams containing a $W^-$ in the $s$-channel. The $\pi^-$ and $W^-$ have angular momentum $0$ and $1$ ...
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1answer
139 views

Is it useful to geometrically represent conservation laws?

In my physics class, we just studied collisions. We learned that momentum is conserved in collisions. I decided to examine a one dimensional collision in which two objects collide - object 1 and ...
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3answers
489 views

Conservation of energy and continuity equation

When physicists say energy is conserved, do they mean that energy satisfies the continuity equation: $$\triangledown \cdot j+\dot{\rho}=0$$ On the internet there is plenty of talk of how the ...
49
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6answers
3k views

Is there a way for an astronaut to rotate?

We know that if an imaginary astronaut is in the intergalactic (no external forces) and has an initial velocity zero, then he has is no way to change the position of his center of mass. The law of ...
2
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1answer
177 views

Conservation of Hamiltonian vs Conservation of Energy

What is the difference between conservation of the Hamiltonian and conservation of energy?
5
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2answers
85 views

Conservation of ang. momentum for paths reaching a rotation axis

My question is the following: if we had the trajectory of a particle eventually reaching a point of a rotation axis $ \vec{u} $ (take that as being the z-axis for convenience) by an angle $ s $, ...
4
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4answers
358 views

Detecting a photon without changing it: Does it break conservation laws?

This is about an article published on ScienceMag: Nondestructive Detection of an Optical Photon. I don't have access to full text, but you can see a brief transcription in this link. Basically, it ...
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1answer
289 views

Deriving $p = mv$ from translational symmetry (momentum conservation law)?

"In classical mechanics, momentum is defined as the quantity which is conserved under global spatial translations or, alternatively, as the generator of spatial translations." (G.Parisi, ...
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2answers
232 views

Tension in vertical circular motion

In vertical circular motion we conserve energy for calculating velocities at a point (if initial velocity given). But, energy can only be conserved when forces are conservative. Tension is not a ...
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2answers
211 views

Confused about elasticity and collisions

I was solving the following problem and the explanation to it confused me. There are two objects with mass $m$ and $M$, respectively. The object with mass $m$ has a velocity of $\sqrt{2gl}$ and ...
-2
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1answer
281 views

How momentum/energy is distributed in a elastic collision? [closed]

We know from conservation of momentum or energy that energy (lets think about one quantity at a time) is conserved before and after collision. But how the energy is distributed between the bodies? I ...
2
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0answers
61 views

Conservation laws in vacuum phase transition

Let's consider a bubble nucleation phase transition between different vacua via quantum tunnelling .For my understanding a particle must penetrate the potential barrier and find herself in an another ...
3
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1answer
2k views

Can the velocity of the center of mass of two spheres change after a collision?

I'm curious as to whether or not the velocity of the center of mass of a system comprised of two spheres can change after the two spheres collide. Looking at the equation for the velocity of the ...
7
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1answer
375 views

Charge neutrality of the Universe: evidences and theories

I've always wondered why the number of protons in the Universe exactly matches the number of electrons. They are such different particles with totally different cross sections. So, first of all, is ...
1
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0answers
251 views

Is total angular momentum conserved in particle interaction?

Imagine that two electrons interact by exchanging a virtual photon. I know that the total energy and linear momentum of the two electrons is conserved by the interaction. Is the total (orbital) ...
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3answers
458 views

conservation of momentum when a bullet hits a block

why momentum is conserved when a bullet hit a block horizontally even when force of bullet is acting on it and net external force is not zero ?
1
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2answers
585 views

The physical observation of the conservation of energy?

Aside from Noether's Theorem, how do we know energy is conserved? Energy is the capacity of a system to do work. It's the number that tells me how much "force" a system can apply over a distance. For ...
2
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3answers
9k views

Kinetic energy and momentum conservation in an explosion?

My physics book says, "A firecracker sliding on ice has the same total momentum before and after it explodes." I understand this part. This is because of Newton's 3rd law, and no external forces. This ...
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2answers
13k views

Perfect elastic collision and velocity transfer

So my teacher told me that when you have two identical balls in a perfectly elastic collision, the first ball A will collide with B and afterwards A will stop and B continue. Why is this? Doesn't ...
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1answer
33 views

Flat space current conservation sign confusion

It is said that in Minkowski spacetime, the current conservation law for the number current $N^\mu$ where $N^0$ is the number density and $N^i, i=1,2,3$ is the particle flux in the $x^i $ direction, ...
3
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1answer
165 views

Ghost Number Conservation

I've been reading about gauge theory quantization, and understand it mostly. The only thing I don't get is why people talk about "ghost number conservation". As far as I can tell, the ghost number is ...
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2answers
384 views

naive question on Boltzmann equation and conservation laws

The Boltzmann equation in absence of external force reads: $\frac{\partial f}{\partial t} + \vec{v} \cdot \frac{\partial f}{\partial \vec{r}} = \left( \frac{\partial f}{\partial t}\right)_{coll}$ ...
2
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0answers
89 views

Spin of a decay product

A particle A decays into particles B, C and D. The spin of A, B and C particles is 1/2 each. What are the possible spins of particle D? My attempt is the following: Since B and C have spin 1/2 ...
4
votes
1answer
232 views

Question on conserved quantities and Noether's theorem

I have a question about Noether's theorem in the context of QM, which I'll state in the context of the weak interaction but the basic point could be generalized. According to Noether's theorem, given ...
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0answers
126 views

Collision of 2 neutrons

If two neutrons collide in 3D space and we want to determine the final velocities of both nuetrons (3 components for each neutrons), we can use the conservation of momentum equations and the ...
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1answer
229 views

Derive conservation law using divergence theorem

Material scientists have discovered a new fluid property called "radost" that is carried along with a fluid as it moves from one place to the next (just like a fluid's mass or momentum). Let ...
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1answer
1k views

Are principle of Conservation of energy and principle of conservation of momentum consequences of Newton's laws?

It is known that principle of Conservation of momentum and principle of conservation of energy are two fundamental principles of physics.But in RP Feynman's Lectures of physics, in the chapter of ...
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1answer
127 views

Is it possible to project a problem of mechanics in a lower dimensionality?

I had the intuition that, in classical mechanics, when the trajectory of a body is known, then analysis of its motion can be done in the linear space of that trajectory, if all forces are projected on ...
4
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1answer
243 views

Why does the pion half-life differ between the charged and uncharged species?

Why does the uncharged pion have much shorter half-life than the charged pion despite the fact that the uncharged pion has a little bit less mass than the charged one, so that according to the ...
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1answer
213 views

Motion in a central field and angular momentum

Is it correct that for a motion in a central force field, e.g. a gravitational field, the absolute value of the total angular momentum of the particle and the component of the perpendicular to the ...
0
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1answer
150 views

Does a fundamental principle require specific concepts? [closed]

The angular momentum principle is a fundamental principle. So it can explain a large variety of phenomenon. Doesn't it need concepts like center of mass also for explaining phenomenon? Or just the ...
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0answers
353 views

Penrose's Zig-Zag Model and Conservation of Momentum

I was reading through Penrose's Road to Reality when I saw his interesting description of the Dirac electron (Chapter 25, Section 2). He points out that in the two-spinor formalism, Dirac's one ...
2
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2answers
741 views

Conservation of linear momentum magnitude along a trajectory

I was once criticized for "taking angular momentum as momentum going in a circle". I was loosely trying to state, in classical mechanics, that in using conservation of momentum, one can switch between ...
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2answers
4k views

Why can't we destroy energy?

From a wikipedia article: In physics, the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither ...
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1answer
112 views

How is momentum conserved when a magnet attracts a metal?

Suppose your have any magnetic object and no external force acts upon it, and the object comes near a metal which causes an impulse (think that will happen). However, the magnetic force is internal to ...
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3answers
674 views

Is it possible to deduce the conservation of angular momentum from the conservation of energy?

Is it possible to deduce the law of conservation of angular momentum from the law of conservation of energy? If possible, by what sense the conservation of angular momentum has the status of law, if ...
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2answers
740 views

Accretion disk physics - Stellar formation

I was going through the Wikipedia page for Accretion disks, and I couldn't comprehend what the meaning of this is: "If matter is to fall inwards it must lose not only gravitational energy but also ...
1
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1answer
106 views

Does the non-relativistic conservation law of particles have an underlying (approximate) symmetry?

In momentum and energy is low enough, we end up with the same number of neutrons, protons and electrons after a collision as before it. This can be considered an approximate conservation law. ...
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2answers
485 views

Law of conservation of energy and kinetic energy: deformation of a spring [closed]

Problem: A block B of 1,5 kg is attached to the right of a spring (not deformed, with its right side attach to a wall) with a constant of $k = 80 N/m$ and, at rest, the block enter in collision with ...
4
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2answers
821 views

Symmetry of Euler-Lagrange equations and conservation laws

Continuous symmetry of the action implies a conservation law, but what if equations of motion have a continuous symmetry? Does it imply a conservation law? Also is symmetry of equations of motion ...
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1answer
110 views

Question about Cars: Momentum

Car B rests at the bottom of a frictionless inclined plane. In order to travel a height of 0.6m and maintain a speed of 2 m/s at the end of the track it needs to start with 4 m/s. a) If ...
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0answers
100 views

Motion on a smooth surface

A particle of mass $m$ is moving on the inner side of smooth circular cylinder of radius $R$ whose $Oz$-axis is vertical and directed downwards. The particle started its motion from the $x$-axis with ...
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1answer
113 views

Conservation of energy of a rotating body [duplicate]

The famous example of acrobats shrinking their bodies to increase their rotation speed is well known. Where does the energy to increase the speed of their rotation comes from?
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2answers
100 views

Conservation of momentum

I've got following problem. There are $N$ particles in an isolated system. The equation of motion for a particle $i$ is ...
0
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1answer
194 views

Theoretical considerations on the conservation of energy and the conservation of linear momentum

I report to you an interesting excerpt from my Physics book. It is an Italian version, so I apologize in advance, as I'm sure I won't give proper justice to its beauty in the translation as the ...
2
votes
1answer
147 views

Momentum paradox [duplicate]

A cistern rail car is standing on infinitely slippery ice. The cistern is filled with water and it has an outlet in the form of a thin vertical pipe (spout) at the left end, so when the valve is open ...
8
votes
2answers
6k views

Conservation Vs Non-conservation Forms of conservation Equations

I understand mathematically how one can obtain the conservation equations in both the conservative $${\partial\rho\over\partial t}+\nabla\cdot(\rho \textbf{u})=0$$ ...
2
votes
0answers
371 views

Charged pion decay and spin conservation

Charged pions $\pi^\pm$ decay via an intermediate $W$ to (e.g.) a lepton-neutrino pair. The pions being scalar (spin-0) particles and the intermediate $W$ having spin 1, how is spin conserved in ...
0
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1answer
81 views

Why do cosmic bodies revolve? [duplicate]

Why do cosmic bodies such as planets, stars, satellites revolve? What made them to revolve after the formation of universe?
2
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2answers
167 views

How is the Principle of Conservation of Momentum proven using the Momentum-Impulse Principle?

Consider two particles moving in the same direction on the same line, $A$ and $B$, with mass $m_A$ and $m_B$, respectively. They also have velocies $u_A$ and $u_B$. They collide. After the collision A ...