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

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Hamiltonian Noether's theorem in classical mechanics [duplicate]

How does one think about, and apply, Noether's theorem in the classical mechanical Hamiltonian formalism? From the Lagrangian perspective, Noether's theorem (in 1-D) states that the quantity $$\sum_{...
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Energy conservation limited by uncertainty principle

The way I learned it from practicing Fourier analysis and signal processing besides quantum mechanics, is that Energy conservation cannot be achieved in short time scales, and that limits energy ...
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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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129 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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204 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} : (u\...
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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
813 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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3answers
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Why is momentum conserved in an inelastic collision and kinetic energy is not conserved? [duplicate]

We know that in an inelastic collision that total momentum of the system before collision equals the total momentum after collision. But total kinetic energy before collision is not equal to total ...
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509 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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160 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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278 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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71 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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555 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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432 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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144 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
163 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 anti-...
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122 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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1answer
83 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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How can a planet gravitationally capture objects?

I would expect that any asteroid or other object originating far away but passing near a planet would pick up speed and energy as it approaches, but unless it comes into contact with the atmosphere (...
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72 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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68 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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3answers
570 views

Two dimensional elastic collisions with varying angle of incident

If in an elastic collision I know all initial values and that mass for each object remains constant throughout the collision (but different from one another) how can I determine their final velocity ...
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1answer
161 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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165 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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253 views

Collision of Discs and Snooker Kicks

I woke up this morning thinking about spinning discs. Could someone verify whether my reasoning below is correct? Problem 1 Suppose have two identical uniform discs constrained to move in a plane. ...
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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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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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295 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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1answer
292 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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69 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 slide....
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1answer
279 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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Can conservation of momentum be violated?

The law of the conservation of momentum has been established for hundred of years. Even in Quantum field theory every particle collision must be momentum-conserving if there is homogenity in space. ...
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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
273 views

Shooting a bullet at a system of blocks [closed]

So, I made this question up myself.... and I'm curious about the answer. It requires only secondary-school-level knowledge of physics: You have a surface (ground) with a certain coefficient of ...
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1answer
268 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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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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212 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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Confusion regarding rotational motion!

Let us assume I have a rod of some mass m, moment of inertia I, length l and center C. If I apply a force F on C for a duration of time t, it will accelerate forward. If I apply it elsewhere, the ...
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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
138 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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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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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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Does a particle annihilate only with its antiparticle? If yes, why?

Or to put the question another way - what is the result of a proton-positron collision, or an up quark-charm antiquark collision, etc.? As far as I know, annihilation happens only between particles of ...
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1answer
105 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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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
440 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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1answer
459 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
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Kepler problem in time: how do two gravitationally attracted particles move? [duplicate]

Two particles with initial positions and velocities $r_1,v_1$ and $r_2,v_2$ are interacting by the inverse square law (with G=1), so that $$ {d^2r_1\over dt^2} = - { m_2(r_1-r_2)\over |r_1-r_2|^3} $$ ...
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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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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 ...