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

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42 views

Ball collides with another. Find velocities [closed]

A ball moving at a velocity of 5.0 m/s east when it collided with an identical ball at rest. Both balls are the same mass. The first ball moved at 60 degrees north of east. The second went 30 ...
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2answers
80 views

Can a spinning object stop spinning?

Suppose there's an object spinning in empty space. That object has mass and angular momentum. Is it possible for that object to stop spinning without any external intervention, say due to a chemical ...
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103 views

Where do the energy and momentum come from when light is perfectly reflected on a surface?

If you send a light beam to a surface which reflects the light perfectly I would say that on the surface is put a force of the light so the surface moves backwards. But where does that energy comes ...
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3answers
126 views

Why is $\mathbf \nabla \cdot \mathbf {J} = 0$ where $\bf J$ is the current density?

This is quoted from Purcell's Electricity & Magnetism: ...if charge forever pours out of, or into, a fixed volume, the charge density inside must grow infinite, unless some compensating charge ...
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1answer
333 views

Conservation of momentum with MOND

Assuming as true the phenomenological MOND law for low accelerations ($< 10^{-10} m s^{-2} = a_0$), and considering a small mass $m$ attached to a larger mass $M$ by a faint spring (let's think of ...
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64 views

How is water falling from a tap an example of the conservation of mass?

In a book, Conceptual Physics by Benjamin Corral, it was given that when water falls down from a tap, it is a case of conservation of mass. They said that this is because water is thick when just ...
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2answers
92 views

What was the initial source of energy?

According to law of conservation of energy; "Energy can neither be created nor be destroyed but may change it's form" Where did the energy come from which led to the creation of universe by big bang?
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1answer
143 views

Why is the decay of a neutral pion $\to$ electron-positron loop suppressed?

To my understanding the decay of a neutral pion into an electron-positron pair can only happen by the electromagnetic force and the mediation of two virtual photons in a triangle-diagram, so it is ...
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7k 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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113 views

Conservation of momentum and conservation of energy

I had the impression that conservation of momentum is valid only when we consider no loss of kinetic energy into heat etc. My tutor says that even if we have loss of kinetic energy as heat in a ...
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0answers
21 views

How can intuitively guess what conserved quantities has the system that I am studying?

I'm taking a course in Classical Electrodynamics and in one problem my teacher introduced us to a triplet of fields ($\phi^a$) invariant under internal rotations, i.e. transformations like: ...
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2answers
1k views

Difference between weak and strong interactions?

This was a statement given in my class: "Strangeness is conserved in the strong and electromagnetic interactions, but not in a weak interaction " But could someone please tell me how we ...
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5answers
4k views

Why Does Angular Velocity Increase as Radius Decrease?

Suppose a child were to ask you why a tetherball (picture below) seems to speed up as it wraps around the pole. How would you explain this to them? Certainly you wouldn't say something like, "Angular ...
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40 views

Is the conservation of probability in the Schroedinger's equation unique?

The Schroedinger's equation can be viewed as a diffusion equation with imaginary constants $a$ and $b$ satisfying, $$(1) \quad \Psi_t=a \cdot \Delta \Psi-b \cdot V(x,t) \cdot \Psi$$ However if $a$ ...
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2answers
70 views

When a ball bounces on a wall at an angle, why does the angle of incidence equal angle of reflection?

When a ball is thrown to a wall without spin and bounces, supposing it's an elastic collision, why does the angle of incidence equal the angle "reflection" (relatively to the normal), why isn't it any ...
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0answers
48 views

Continuity equation - water mass conservation

I have a problem to understand a passage in the article "Modeling the dynamics of pressure propagation and diameter variation in tree sapwood" that you can find here: ...
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0answers
43 views

Higher order versions of momentum? Can conservation principles be established and used? [closed]

Question Can higher order derivatives of momentum be useful in creating theories of dynamics if they have conservation principles? Even if they aren't needed, could it be done in theory? For ...
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1answer
62 views

Nail clippers and conservation of momentum

As I clip my fingernails using nail clippers, pieces of nails sometimes fly off violently, reaching the other end of the room. What's happening in terms of conservation of momentum, how are the pieces ...
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1answer
91 views

conservation of momentum? [closed]

At hyperphysics I got this image, with the same description in text as is in this image It says that when a massive particle (say $A$) moving with a velocity collides with an object having a ...
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1answer
89 views

Newton's third law at the quantum level? [closed]

let's look at force at the atomic level to understand the newtons third law of motion. I'll use Helium atoms as an example. Now imagine we start with one atom HE2 stationary, and throw another atom ...
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1answer
59 views

Conservation of momentum always? [closed]

Consider two atoms A and B, with A at rest and B in motion towards A. As soon as B is in the atomic field of A, will it stop with its energy going into B or will it just move up to a postion where ...
3
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1answer
65 views

Second Kepler's law implies that constant velocity

I was trying deduce that if we suppose that the planet's orbits are circular and de Kepler's secong law is true: A line joining a planet and the Sun sweeps out equal areas during equal intervals of ...
5
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1answer
98 views

Why doesn't the 9th ball move in the break in the nine-ball pool game?

In the game of nine-ball pool, we break the rack like shown below: In the break, we hit the 1st ball with the cue ball. Many people familiar with pool games say that if the rack is constructed ...
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2answers
44 views

Why can't a force between two particles exert a torque?

Let's say I have two particles which exert a mysterious force between them. Say they lie on the x-axis. Newton's 3rd law tells us that the force on the two particles due to the other must be opposite ...
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1answer
94 views

Mathematical proof of an electron cant absorb a photon [duplicate]

How can we mathematically prove that a free electron cant absorb a photon totally?
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23k views

Calculating force of impact

Since $\text{force = mass}\times\text{acceleration}$, is it right to say that an object traveling at a high constant velocity (zero acceleration), exerts zero force upon impact with a stationary ...
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3answers
3k views

Conservation of 4-momentum in special relativity

I understand that the inner product of two 4-vectors is conserved under the Lorentz transformations, so that the absolute value of the four momentum is the same in any reference frame. This is what I ...
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2answers
163 views

Multiple conservation of momentum in one [closed]

I am in a Grade 12 Physics course and my teacher gave this question for bonus marks, to be solved with only highschool concepts. I suspect that this problem has to do with conservation of momentum, ...
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2answers
167 views

Why are gravity and spring-force conservative forces?

I know that the amount of work does not depend on the followed path but on the position when speaking about conservative forces. But could someone please prove/explain why exactly those two are ...
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1answer
58 views

How does potential energy relate to kinetic energy on a rollercoaster?

This is appearantly how the potential and kinetic energy are in the different positions of the rollercoaster, if I am not mistaking. However I don't understand why the total energy is energy and ...
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1answer
38 views

Isolated system and external forces

I have problem of understanding fully "isolated system", if i imagine a car moving at a constant speed ,can I say that is isolated system although an interna force (car's engine) cancel out an ...
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2answers
115 views

Violation of the Newton's first law of motion in the movement of spacecrafts in the vacuum of space [closed]

Suppose a rocket at rest in space where there is a complete vacuum and no force to influence the rocket: In the left we have the rocket with a metal ball in it. After the ball is thrown out of the ...
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2answers
124 views

Why does Conservation of Momentum fails because of friction? [closed]

I did this experiment in my lab. We had two cars with wheel connected by a string. One car has a spring that can be triggered by a switch at the top of the car. (Frictionless surface and light string$ ...
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2answers
39 views

Why does current density changes but not current? [closed]

let us assume that we have a conductor with a specified resistance ( case 1 ) and a normal conductor ( case 2 ) as shown in the figure , and now we apply an external electric field with a battery on ...
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0answers
14 views

Relativistic particle decay in ZMF [closed]

This is a problem that I feel is harder than I am making it but say you have: A particle at rest of mass $M$ decays into two smaller particles of masses $m_1, m_2$ where $m_1 \neq m_2$ and $M > ...
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3answers
109 views

Conservation of Momentum

Conservation of momentum can be derived during collisions by using Newton's laws of motion. But in other cases, do we simply take it like an axiom ?
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1answer
179 views

What's the corresponding symmetry of enstrophy conservation?

In fluid mechanics, especially 2D turbulence study, people talk about conservation of enstrophy. But I can't really understand enstrophy very well, and what's the corresponding symmetry of enstrophy ...
3
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1answer
79 views

Are there conserved quantities in field theory which don't arise from Noether's Theorem?

In some QFT texts one writes down the number operator $N$ for free theories, such that when acting on an $n$-particle state $|n\rangle$ we have $$N|n\rangle=n|n\rangle$$ In free theories this is a ...
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1answer
234 views

How does one account for the momentum of an absorbed photon?

Suppose I have an atom in its ground state $|g⟩$, and it has an excited state $|e⟩$ sitting at an energy $E_a=\hbar\omega_0$ above it. To want to excite the atom, one generally uses a photon of ...
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0answers
59 views

Does a planet's orbital angular momentum affect its rotational angular momentum?

For example: If the moon was closer to the earth, assuming the orbital momentum was conserved and not worrying about earth's rotation, would the moon's rotation rate be effected?
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2answers
107 views

Conservation of angular momentum while sitting on a spinning chair

Today my friend was sitting on a spinning char. By moving his top part of the body left to right and his bottom part of the body the opposite he managed to spin. As I understand Conservation of ...
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1answer
100 views

Conservation of angular momentum during rolling

A disk having initial angular velocity $\omega$ is gently placed on a rough horizontal surface. What is the angular velocity of rotation when pure rolling starts? I've tried applying conservation ...
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1answer
47 views

Help with my bouncy ball lab (I know the factors just not how to approach them) [closed]

In my physics lab we need to determine the factors that account for the energy "loss" during a high bounce ball bounce. I know that energy is "lost" (not really) to heat, air resistance, and sound. ...
5
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3answers
141 views

How is the conservation of momentum satisfied in long-range attraction such as electromagnetism and gravity?

I'm not a physicist, but my understanding is that electromagnetism (including attraction between opposite charges) is mediated by the photon, and gravity is probably (hypothetized to be?) mediated by ...
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0answers
36 views

Gravitons, photons and conservation of momentum

How can gravitons be emitted from a mass to cause an attractive force to another mass? The same question could be asked of attractive e-m forces as well. Don't these violate the conservation of ...
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3answers
104 views

A moving bus suddenly stops. Is its momentum destroyed?

Absolutely not but how does momentum transfer to surrounding (ground, air particles)?
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1answer
296 views

Proving the conservation of 4-momentum for a particle collision $A+B\to C+D$

Let me say that particle A hits particle B and two particles come out - C and D; In system S I can write: $$p_A^μ+p_B^μ=p_C^μ+p_D^μ;\tag{1}$$ here $p_N^μ$ is the 4-momentum. Using the Lorentz ...
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1answer
2k views

What happens if object is thrown in empty space?

If I throw a object in empty space, I apply a force to throw that. Then it gains some acceleration and it's speed increases. So will it's speed keep on increasing, or it will get stable? If yes, ...
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1answer
74 views

Proving that Killing form contractions with geodesics are constants of motion

I want to prove the fundamental theorem of Killing forms, namely that $$\frac{d}{d \lambda} \Big( \frac{d P^{\mu}}{d \lambda} \xi_{\mu}(P(\lambda)) \Big) = 0 $$ If $P(\lambda)$ is a Geodesic ...
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162 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 $$\mathscr L_g=\frac 1{16\pi k}\sqrt{-g}R.$$ ...