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

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273 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
68 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
545 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
426 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
142 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
161 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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2answers
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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0answers
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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2answers
64 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
157 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
160 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
281 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
268 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
499 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
67 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
171 views

Is my proof of the thought experiment that Walter Lewin proposed in lecture 16 valid? [closed]

A tennis ball bounces off a wall elastically. The momentum of the wall changes, but the kinetic energy of the wall remains zero. How is that possible? Walter Lewin Lecture 16 - Ball bouncing on wall?...
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1answer
272 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
189 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 ...
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3answers
365 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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2answers
267 views

Why do particles of equal mass (with one at rest) undergoing elastic collisions scatter at only right angles?

This is from the Section 9.6, page 351 of "Classical Dynamics of Particles and Systems" by Thornton and Marion. By setting a up a system where mass 1 has initial momentum $m_1 u_1$ and mass 2 is at ...
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1answer
264 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
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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1answer
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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1answer
66 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
112 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
137 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
2k views

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
103 views

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
115 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
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
181 views

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
426 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
62 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
435 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
159 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
101 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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2answers
65 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
715 views

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
329 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 ...
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1answer
76 views

Continuity Equation for Momentum

Momentum is a conserved quantity, which makes me wonder if we can write an equation for the local conservation of momentum in the form of a continuity equation. If we're considering a system of ...
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3answers
288 views

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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1answer
221 views

Physics simulator based on conservation laws?

Reading the article: http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Relationship_to_the_conservation_laws there's a section stating that: ...
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3answers
163 views

How can I find the angular and linear velocity of a 2D body that breaks into two bodies?

Afternoon. This is my first question, so do let me know if I'm doing anything wrong. Looking for help on building a 2D physics game engine with bodies that split in half: I have a two dimensional ...
25
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5answers
13k views

Is it possible to shoot bullets in space or would the recoil of the gun be too strong?

I've read a few articles that say that astronauts have already brought guns in space and that shooting bullets in space is possible. But won't the recoil of the gun be too strong? Law of ...
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3answers
347 views

What is actually a conservation law?

Though in his lectures, Feynman didn't define conservation law, he did use it while explaining divergence theorem: [...] heat is conserved. That is, no heat is generated inside the material and ...
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80 views

Antimatter universe and Noether's theorem

I am studying Feynman's "symmetry in physical laws", where he talks about conservation laws for corresponding symmetries. (I know this is Noether's theorem, I am studying this from David Tong's ...
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0answers
26 views

Interpretation of Mass Continuity Equation in MHD [duplicate]

I'm writing up my final-year dissertation and I'm required to give, as part of the introduction, an analysis of all the equations (and their terms) of which I use. Embarrassingly, whilst of course ...
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1answer
62 views

Time evolution of generalized angular momentum operator

We define this operator : $$M^{\mu\nu} = \int d^3x~(x^{\mu}T^{0\nu} - x^{\nu}T^{0\mu})$$ where $T_{\mu\nu}$ is the energy momentum tensor (see e.g. Energy momentum tensor from Noether's theorem) ...
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1answer
155 views

Finding final velocity in inelastic collision [closed]

Information: In a shipping company distribution center, an open cart of mass 49.0-kg is rolling to the left at a speed of 5.40-m/s (see the figure). You can ...