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

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Derivation of the kinetic energy of decay product [on hold]

By conservation of momentum: $$M_{\alpha }V_{\alpha }+M_{Y}V_{Y}$$ By the conservation of energy: $$(\frac{1}{2})M_{\alpha }V_{\alpha }^{2} + (\frac{1}{2})M_{Y }V_{Y }^{2}$$ Here I begin the ...
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Collsion in 2D, find angle and final momentum

Suppose that the collision is completely inelastic. Disregard friction as the surface is smooth. Suppose that one is in the direction of $(1,0)$ and the other $(0,1)$. Is the final momentum the ...
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1answer
23 views

Greater momentum than initial?

The question is : Heavier object A, initially at rest, is struck by lighter object B. Is it possible for object A to have a larger final momentum than the initial momentum of object B? The ...
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34 views

One extra conservation equation or angle relation for a relativistic collision

Edit: It seems that the derivation for the center of momentum frame was incorrect, due to the fact that the center of momentum velocity changes after the collision. This is also true for the ...
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1answer
46 views

Particle Physics Decay Question - Eta Prime Decay Parity/Angular Momentum Conservation

I was hoping someone could clarify why the following decay does not occur: $ \eta ^{'0} \rightarrow \pi ^{0} + \rho ^{0}$ The quark compositions and spin parity are as followed: $ \eta ^{'0} : ...
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32 views

Conservation Laws and Isotropic Time

In most dynamics books I've read they refer to conservation laws and their associated symmetries. I know that the conservation of momentum is a result of the homogenity of space angular momentum is ...
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30 views

Potential (which only depends on the length of a position vector) [closed]

a) A potential $U$ only depends on the length $r=|\vec{r}|$ of the position vector $r$. Show that $$\vec{\nabla}U(r)=U'(r)\frac{\vec{r}}{r}. $$ What properties does this vector field have? ...
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0answers
28 views

What are the parity of particles? [duplicate]

When looking to see if particle collisions/decays are possible and what force they act through, how do you know the parity of particles to know whether they act through weak force? Is there a grouping ...
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2answers
84 views

How does the kinetic energy of a ballerina increase? [duplicate]

When a ballerina pulls her arms in, her rotational kinetic energy increases because angular momentum is conserved. That means that work must have been done on her. I saw somewhere that there is work ...
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0answers
19 views

Covariant derivative of Noether current [closed]

I am working with a non-abelian gauge gauge theory that has one gauge field and a complex scalar field. I am supposed to prove that \begin{equation} (D_\mu j^\mu)^a=0, \end{equation} where ...
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1answer
54 views

Relativistic momentum of an electron

Suppose we have some interaction between a photon and an (initially) stationary electron, and we wished to find the final momentum of the electron. Should we solve this using conservation of momentum, ...
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1answer
44 views

After a glancing collision, why do air hockey pucks spins around in circle?

For a lab testing the conservation of momentum, I had to hit an air hockey puck so that it would hit another stationary puck in a glancing collision. After the pucks collided, they travel their ...
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1answer
36 views

Where would be the source of thrust for a rocket thruster in space?

I am making a game that involves spaceships and I was building an engine block. That engine block spawns a "thruster fire" under it that represents the...thruster fire. My question revolves around ...
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2answers
141 views

Is there a quark conservation law?

The section on particle interactions in my revision guide says that only the weak interaction can change quark types, e.g. when a neutron changes to a proton the down quarks in the neutron are changed ...
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1answer
27 views

How to tell if a particle interaction is strong/weak?

Problem: Prove that the reaction $p+p\rightarrow p+K^+$ is impossible, where $p=\text{proton}$, $K^+=\text{kaon}$. (Side question: Is there a specific name for a positive Kaon, just like a "positive ...
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1answer
46 views

speed of light and conservation of linear momentum

Acoording to the results of Michelson–Morley experiment, the speed of light is constant regardless of the velocity of the source. Now, I want someone to correct me if I am mistaking about the ...
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3answers
62 views

Does (mass $\rightarrow$ pure energy) conversion need both matter $and$ antimatter?

A question in my revision guide: "Explain why the mass of a tree cannot be converted directly into energy." The answer explains that the tree contains particles but not their corresponding ...
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2answers
47 views

Do inelastic or elastic collisions have any effect on a large bodies movement (like the Earth)

Recently read a question regarding the temperature of the CMB & one of the comments got me thinking... From what I understand, collisions between subatomic particles and objects out in ...
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0answers
49 views

What do the recent vacuum tests mean for the believability of NASA's EM Drive? [closed]

NASA has been testing an EM drive for a while. Although the initial results appeared positive, the scientific community was skeptical because these results would violate conservation of momentum. Much ...
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2answers
32 views

Inelastic Collision and Energy

How is is possible for momentum to be conserved and for KE to not be conserved? Momentum is related to velocity and velocity is related to KE, therefore if KE was lost, how can the system have the ...
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1answer
98 views

Quantum violation of Newton's Third Law? [closed]

From this site: http://www.learning-mind.com/5-thought-provoking-quantum-experiments-showing-that-reality-is-an-illusion/ I gained the knowledge that a group of scientists, upon measuring a tiny ...
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2answers
101 views

In the time of the dinosaurs the Earth rotated once in 17 hours rather than about 24 hours, where did the rotational energy and angular momentum go?

where did the rotational energy and angular momentum go? Some claim that the angular momentum went to the Moon. Astronauts put a corner reflecting mirror on the moon and reflected a Laser and timed ...
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2answers
72 views

can kinetic energy be independent of mass.?

Why is it said that the kinetic energy acquired by a body of after traveling a fixed distance from rest under the action of constant force is independent of mass? Nd yeah the mass of the body is ...
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4answers
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Why do electrons, according to my textbook, exist forever?

Does that mean that electrons are infinitely stable? The neutrinos of the three leptons are also listed as having a mean lifespan of infinity.
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1answer
48 views

Newton's Third law of motion: A hammer hits a wall vs. hits a tire

When we hit the wall by a hammer, Newton’s third law says that hammer applies force on the wall (action) and wall applies force on hammer (reaction) equal in magnitude but opposite in direction ok ...
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2answers
90 views

Where does the angular momentum of the solar system come from? [duplicate]

We inhabit a system with significant angular momentum: http://www.zipcon.net/~swhite/docs/astronomy/Angular_Momentum.html If our solar system formed by gravity gathering its material together to ...
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1answer
40 views

Newton's law for rigid bodies

Is newton's third law valid for rigid bodies? Say, if we have a bullet hit on a rod which is vertically placed on a rough floor. Friction is just sufficient sp that the rod rotates and does not ...
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1answer
55 views

Charge conservation in the complex Klein-Gordon Field

This is an extremely naive question (based on a knowledge of chapter 2 of peskin and schroeder) so apologies for any things that seem obvious. The complex scalar field, when quantized, has a conserved ...
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2answers
92 views

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

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 ...
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3answers
82 views

Constants of motion in quantum mechanics

What is the meaning of a constant of motion in quantum mechanics (an observable-operator that commutes with the Hamiltonian) in contrary with classical mechanics?
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1answer
64 views

First-order EM Feynman diagram?

Is there any 1st order electromagnetic Feynman diagram? I.e. a process whose probability is just $\propto \alpha_{EM}$? If not, is there any physical reason why? We always need at least two particles ...
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1answer
74 views

using tetrads to glue local currents into global currents

According to John Baez it is possible to take a locally conserved tensor $\nabla_\mu\: T^{\mu\nu}(x)=0\ \ \ \ \ \mbox{(locally)}$ and convert it to a globally conserved tensor by "patching" ...
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1answer
151 views

How did Feynman prove that energy cannot be extracted from electric field?

In the Feynman Lectures, vol. II, chapter 4, Feynman discusses electric potential and says: If we carry a charge from point $a \to b$, $$W = -\int_{a}^{b} \mathbf{F} \cdot ds.$$ Now, in general, ...
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1answer
50 views

What justifies conservation laws in non-uniform spatial/temporal fields, if Noether's theorem doesn't?

Noether's theorem is based on the assumption that the Lagrangian is independent of position/time/angle/etc. Does this mean it doesn't prove, for example, conservation of momentum in a gravitational ...
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1answer
56 views

Difference between speed of light and of bullet after passing through a barrier [duplicate]

I just read that, when a ray of light traveling in vacuum at $c$ strikes a glass slab, its speed decreases and then when it re-emerges it gets back to its original speed i.e $c$. If I draw a ...
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1answer
58 views

Using Conservation of momentum and Energy to solve a problem [closed]

A 9kg bullet is fired horizontally into a 10 kg block of wood suspended by a rope from the ceiling. The block swings in an arc, rising 6mm above its lowest position. Find the velocity of the bullet. ...
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6answers
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Is (rest) mass conserved in special relativity?

I don't understand why it is said that the (rest) mass of a system is not conserved in relativity. I mean, the momentum of a system is conserved (i.e.: it remains constant in a frame of reference ...
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4answers
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Sun and planets orbit each other

Do not the planets and the Sun revolve in orbits around each other and the shape of the orbit depends on where the center of gravity of the system is? The greater the mass of the Sun, the closer the ...
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2answers
93 views

Why is $p_\phi$ conserved in a Schwarzschild orbit?

This arises from the question What is the relationship between $a$ and $m$, which I'm afraid I answered just by looking it up in Schutz's book. However Schutz (as he frequently does) glosses over ...
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1answer
62 views

Pair-annihilation why does it occour? [duplicate]

Why does pair annihilation occur with particles and only their matching anti-particle? E.g., electrons and positrons, but not protons and positrons? What is the difference?
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2answers
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Pass to globally conserved currents from locally conserved currents in curved spacetime

Let us begin with a Lagrangian of the form $$\mathscr L= \frac 12 \sqrt{-g}g^{\mu\nu}\partial_\mu\phi(x)\partial_\nu\phi(x)+\mathscr L_g,$$ where $$\mathscr L_g=\frac 1{16\pi k}\sqrt{-g}R.$$ ...
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1answer
60 views

Collisions between an object and a wall

Is momentum conserved when an object bounces back against a wall? The wall doesn’t move, but the object moves in the opposite direction. Assume this is an ideal, elastic collision. If, initially, the ...
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1answer
44 views

Two Bodies Orbiting Around Each Other and Kepler’s Laws?

If two bodies are orbiting around a central center of gravity, how does Kepler’s first law (the one regarding the ellipse) apply?
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1answer
375 views

Why can't Compton scattering happen in leading order of perturbation theory?

Why is the matrix element of Compton scattering in leading order of perturbation theory equal to zero? Why can this process only be described in second order of perturbation theory, i.e. with exchange ...
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2answers
90 views

Comparing the orbit radius of two spherical objects [duplicate]

Assume the mass of star 2 is 4 times the mass of star 1. Compare the radius of the orbit of star 1 to that of star 2. Possible answers: R1:R2=1:4 R1:R2=1:2 R1:R2=2:1 R1:R2=4:1 R1:R2=16:1 ...
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1answer
47 views

Do all “normal” black holes rotate?

Can we assume that most (if not all) black holes are rotating, due to conservation of momentum? I am excluding the micro world from this question, just thinking of the range of stars on the main ...
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0answers
62 views

Noether's first and second theorems

My understanding of Noether's first theorem is as follows. Consider a set of infinitesimal transformations that leave the action invariant, that are indexed by $n$ linearly independent parameters, ...
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2answers
51 views

Eulerian mass conservation on a stream line to Lagrangian mass conservation

if the density of a fluid particle is conserved on a streamline, $$\frac{d\rho}{dt}=0.$$ Why does this mean $$\frac{\partial \rho}{\partial t}+(\mathbf{v}\cdot\nabla)\rho=0$$ is true everywhere? Why ...
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2answers
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Can gyroscope work in zero gravity?

Most ships have two or more gyroscopes to balance on water, man made satellites uses gyroscope for orientation as they fall around earth. All these applications seems to be associated with gravity, ...
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2answers
62 views

Conservation of angular momentum in a planetary system

Why is angular momentum conserved when a planet revolves about sun in an elliptical orbit? Why is linear momentum not conserved in this case? Please use the minimum amount of equations and try to ...