In introductory mechanics, the momentum of a particle is its mass times its velocity. In electrodynamics, the momentum of a field is proportional to the cross-product of the electric field with the magnetic field. In special relativity, momentum is generalized to four-momentum.

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What does it mean to perturb the electromagnetic energy with respect to a change in a Lorentz frame?

I am perturbing the energy of an electromagnetic field using \begin{align} \dfrac{\partial U}{\partial \zeta} = \dfrac{1}{\mu} \boldsymbol{H}\cdot \dfrac{\partial \boldsymbol{H}}{\partial \zeta} + ...
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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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1answer
28 views

Collision and momentum conservation principle

Consider the following problem: A ball is dropped with an angle a between its direction and the normal of the floor and bounces up with an angle b between its direction and the normal of the floor. ...
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102 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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77 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
54 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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3answers
118 views

Why does it hurt more to fall from 10 stories, than 1?

$F=ma$. Falling from any distance, mass stays the same, and acceleration due to gravity stays the same. So, why does it hurt more, the longer you fall?
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44 views

How to calculate linear momentum in this elastic collision? [closed]

A small mass m = 5.0 kg approaches from the left with velocity v and a greater mass M = 40.0 kg approaches from the right with the same velocity. If the collision is elastic and the value of v is v = ...
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2answers
41 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
55 views

What's the physical meaning of change in momentum vector?

If I there is a initial momentum of 10Ns upwards, and final momentum of 10Ns to the right, I can find the difference in momentum by drawing a triangle and finding the resultant vector. But, how is ...
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458 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
53 views

Question on Projectiles and momentum conservation [closed]

Firstly I apologize for bringing forth a high-school physics problem here, as some of you'll find this problem highly childish. But there are certain things i need to clarify about this. Q: An ...
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1answer
50 views

Why two colliding bodies exert varying forces on each other during the course of their collison?

Suppose a bat hits a ball .What is the explanation of why the force is not constant during the course of the collison. At certain instants both ball and bat exert varying degrees of force on each ...
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122 views

What exactly is NASA's proposed mechanism for “propellantless” “EM Drive” propulsion? [duplicate]

Of course, this question runs perilously close to this site's prohibition against discussing non-mainstream physics. However, the accepted answer in meta about what is acceptable and what is not ...
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22 views

Does the force of releasing the latch of a spring-latch contraption affects the force generated by the spring?

There is this contraption in my class, where a rod is attached to a latch and a spring. By pulling the latch back behind a piece of metal, the latch is secured, the rod if pulled back and the spring ...
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35 views

$\mathbf{P}=M\mathbf{v}_{cm}$ for a continuous body?

While restudying some fundamental concepts with greater attention, I have reflected on the following deduction, which I find in my book of mechanics, of the identity of the temporal derivative ...
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2answers
642 views

How do I figure out the momentum of a water balloon when it reaches the person I am throwing it at?

Recently an experiment was performed in which myself and a partner filled a water balloon and threw it back and forth at each other without breaking it. We gradually increased the distance at which it ...
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1answer
72 views

Calculating electrodynamic momentum of a dumbbell (consisting of two point charges) in longitudinal motion

I'm working through a paper on momentum in electrodynamics that requires the integration below and would greatly appreciate any help. I'm pretty sure it evaluates to $2/d$ but I can't quite figure ...
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1answer
159 views

Path integral in quantum mechanics

I am confused by the derivation in Srednicki QFT's chapter 6 from (6.8) to (6.9). In (6.8), we have $$<q'',t''|q',t'>~=~\int DqDp \exp[i\int_{t'}^{t''}dt(p\dot{q}-H(p,q))],\tag{6.8}$$ and ...
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1answer
72 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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36 views

Does conservation of moment hold in space expansion?

I recently watched a video describing that the expansion of space is a "stretch". Everywhere in the universe expanding together. My question is, does conservation of momentum hold true in this ...
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1answer
55 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
71 views

Is angular momentum the conjugate momentum of an angle?

Lagrangian mechanics can be used to describe the double pendulum (see here, for example). In this development are the conjugate momenta $p_{\theta_i}$ exactly the angular momenta $m_i l_i \frac{d ...
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Massless particle that have momentum not equal to 0

I saw some problems in special relativity that use conservation of total energy and momentum and the conclusions are spectacular. My problem is the following: how can a massless particle like neutrino ...
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50 views

How to explain the momentum exchange of two solid particles collision at molecular scale?

Here is my question: assume two solid sphere collided to each other in vacuum, at macroscopic, it can be explain by Newton's law. But at molecular scale, does the surface atoms touch to each other or ...
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1answer
107 views

Why is the position space free particle wavefunction a function of momentum?

This is one of those little things that has always confused me. If someone said to you "in quantum mechanics, the eigenfunctions of a free particle are $\exp(ipx/\hbar)$" how would you know that ...
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How would a plane hitting a ball move (opposite of Pong)?

When a moving ball hits a stationary plane at an angle of incidence to the normal, it bounces away at the same angle (the angle of reflection), which is commonly understood. My question is 1) What ...
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956 views

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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Conservation of momentum in field theory

By studying electrodynamics a while now, I came to this question on the conservation of momentum. In Newtonian mechanics the Third Law allows us to prove the conservation of momentum, with the ...
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1answer
58 views

Relativistic Momentum in One Frame

(I) What is the momentum of a proton traveling at v = 0.75c? This problem requires a simple substitution. Using the equation $\ p_{rel} = m*u * (1/\sqrt(1 - v^2/c^2))$, where $v$ is the speed of a ...
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1answer
230 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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3answers
217 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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114 views

Relativistic Momentum and Energy - Lorentz Transformations [closed]

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab is colliding fully ionized gold (Au) nuclei accelerated to an energy of 200 GeV per nucleon. Each Au Nucleus contains 197 ...
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393 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
74 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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3answers
279 views

Expectation of momentum in the bound state

Is it logically correct to assert that the expectation of the momentum $$\langle \hat p \rangle=0$$ for any bound state because it is bound to some finite region? What is the physical interpretation ...
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2answers
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$p^4$ in radial coordinates not Hermitian

Griffiths' quantum textbook claims in question 6.15 that "$p^2$ is Hermitian, but $p^4$ is not, for hydrogen states with $l=0$." First off, I am puzzled at his use of terminology. An operator is ...
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96 views

Circular motion and momentum

I have to explain why there is a force pointing inwards in a circular trajectory (assume constant speed), but without relying on centripetal force argument. Coordinate system is Cartesian ...
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5answers
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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
65 views

Moving objects using light

Photon do not have mass but they have momentum, can we use laser to pick up golf ball and hurl it several yards away without burning it into crisp?
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1answer
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How to pull out the momentum operator?

In the equation (1.7.17), how does operator $p$ get out of the bracket without any operation though $<a | $, $| x'>$ are function of $x'$? How to prove this?
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Elastic collision of point particle and rod

A 1 meter long rod on the ice with mass $m_2=1$ kg is perpendicularly hit on one end by a point particle with mass $m_1=0.1$ kg. The collision is elastic and the point particle is bounced back in ...
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8answers
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Can we explain Newton's first law mathematically?

At constant speed there is no acceleration. $(f'(x)=v'=0=a)$ .If $a=0$ then $F=ma=0$ and therefore no force acts on the object so the object will continue in the same direction, if any. This is only ...
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1answer
58 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 ...
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1answer
79 views

What is the function type of the generalized momentum?

Let $$L:{\mathbb R}^n\times {\mathbb R}^n\times {\mathbb R}\to {\mathbb R}$$ denote the Lagrangian (it should be differentiable) of a classical system with $n$ spatial coordinates. In the action ...
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2answers
65 views

Can an electron borrow momentum from its field?

Let us consider a charged particle moving with uniform velocity $v$. We know that the EM field due to it has some momentum too. If the mass of the particle is $M$, then the momentum of the particle is ...
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1answer
181 views

Does the velocity vector always point in the same direction as the momentum vector?

I was told that the angular velocity vector does not always have to point in the same direction as the angular momentum vector. This is due to the fact that they are related by the equation $L=I ...
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213 views

Conservation of angular momentum in a free rod

When a collision is elastic and no external torque acts on a system, angular momentum is conserved I found this example and checked the results: A ball (m = 1 Kg , v = p =+22 m/s, Lm = +11, Ke = 242 ...
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Does the spatial momentum of the ground state of a Poincare symmetric QFT vanish?

Consider a flat space QFT, the Lagrangian (in general interacting) has Poincare symmetry, and $\lvert\Omega\rangle$ is the ground state (or just merely no insertion at the far boundaries, from ...