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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Momentum change of a photon upon diffraction at a grating

How is the momentum change of a photon is related to the grating periodicity and diffraction order? I came across with this question while studying surface plasmon polaritons (SPPs) and how to excite ...
4
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2answers
120 views

Is the Einstein Energy-Momentum equation $E^2 = p^2c^2 + m_0^2c^4$ valid only for Free Particles?

Is the energy -momentum relation $$E^2 = p^2c^2 + m_0^2c^4$$ satisfied only by free particles or even bound particles? Does the Energy refer to total Energy(including potential) or only (kinetic ...
3
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3answers
99 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 ...
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2answers
47 views

If one object, moving at a constant speed collides with another object - which is at rest - how much force has been applied?

This is one question for which an answer doesn't really make intuitive sense. In elementary school, we learn F=ma, but where's the "a" in a collision such as this? Edit: Plus, there has to be a force ...
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1answer
63 views

Probability density for momentum in Quantum Mechanics

In a book i found the following equations: $$ \phi(k)=\frac{1}{\sqrt{2\pi}} \int_{-\infty}^\infty \Psi(x,0)e^{-ikx}dx $$ and $$ \Psi(x,t)=\frac{1}{\sqrt{2\pi}} \int_{-\infty}^\infty ...
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2answers
185 views

Why isn't jumping against a wall an elastic collision?

According to this calculator http://www.abecedarical.com/javascript/script_collision1d.html when low mass object hits high mass object it is reflected gaining opposite velocity almost the same as ...
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1answer
94 views

Newton's law of restitution

Newton's law of restitution. Could someone tell me what the easiest form of this law to use is? I usually try to use e=(v(2)-v(1))/(u(1)-u(2)). Does the law also work if I don't know the direction of ...
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2answers
68 views

What is the meaning of SI unit $Ns$ (NEWTON.TIME)?

for ex: when i say newton per meter it mean that 1 newton is applied per 1 meter. but what i mean by saying N.s (s=time in sec) i.e Impulse or momentum. N.s does not mean that certain force is being ...
4
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2answers
107 views

Is there any other momentum besides the Poynting momentum stored in an electromagnetic field?

I am having some conceptual difficulties with energy and momentum stored in the EM field. The force density at a point is $\rho E + j\times B$ Because of conservation of momentum, and because the ...
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0answers
35 views

How did we get rid of the term $\overrightarrow{G}_W$? [closed]

Equation of conservation of momentum: $$\frac{d}{dt}\int_{W}\rho \overrightarrow{u}dV+\int_{\partial{W}}\rho \overrightarrow{u} (\overrightarrow{u} \cdot ...
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2answers
52 views

Does momentum change if rotation radius changes?

Angular momentum is defined as $L = r \times p$ where r is the center of rotation and p is the momentum. Since angular momentum is conserved, if r decreases then p must increase. And since p is m*v ...
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2answers
14k views

Perfect elastic collision and velocity transfer

So my teacher told me that when you have two identical balls in a perfectly elastic collision, the first ball A will collide with B and afterwards A will stop and B continue. Why is this? Doesn't ...
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3answers
81 views

Solar sail, estimate force

I'm trying to get an estimative for how much force is applied on a solar sail by solar radiation. So the first question is elastic or inelastic collision? Elastic colision provides a change in ...
-1
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1answer
120 views

What forces carry hovering objects around the earth with its rotation?

My questions are: a) What significant forces “carry” flying objects around with the rotation of the earth, b) How do each of those forces contribute to that “carrying”, and c) How relatively ...
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2answers
39 views

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 ...
0
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1answer
47 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 ...
0
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1answer
59 views

difference between magnetic dipole moment and torque on a magnetic dipole?

I am a bit confused. What is the difference between magnetic dipole moment and torque on a magnetic dipole ?
2
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1answer
69 views

Relativistic Lorentz Force with constant, perpendiuclar, and uniform E and B fields

I am trying to solve the following problem from the Fundamentals of Plasma Physics by Bittencourt (Problem 2.7): Analyze the motion of a relativistic charged particle in the presence of crossed ...
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0answers
75 views

Why Does Moving a Cup of Coffee Up and Down whilst carrying it Make it less Likely to Spill?

Sometimes I have a Very Full Mug of Coffee and I am trying to carry it without it spilling. I noticed that if I moved the Mug up and down when carrying it, it seemed to settle the coffee and make it ...
1
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1answer
77 views

The proof of a discrete Fourier identity in quantum field theory

On page 25, in the book Quantum Field Theory for the Gifted Amateur by Tom Lancaster and Stephen. J Blundell, it states the following: We impose periodic boundary conditions forcing ...
2
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3answers
493 views

How does the solar sailing concept work?

Wikipedia describes solar sailing as a form of spacecraft propulsion using a combination of light and high speed ejected gasses from a star to push large ultra-thin mirrors to high speeds. I ...
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1answer
29 views

momentum and Kinetic Energy [closed]

Make some general statements about momentum and kinetic energy conservation. So, I is this correct? Two objects of varying mass have the same momentum. The least massive of the two objects will have ...
8
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2answers
570 views

Momentum and spacetime

My apologies up-front for the naive question and my rudimentary understanding, but I should be delighted if someone would enlighten me :) The question has to do with the application of the ...
4
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1answer
67 views

Optimization of Bottle Rocket Water Level

My (entry-level) physics class is building bottle rockets, and we are competing to build the longest-flying bottle rocket. The rockets are filled partway (we get to decide how much to fill them) with ...
0
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1answer
59 views

When can I swap around the order of operators?

I was doing this question: Using $\left< x \middle| p\right> = \frac{1}{\sqrt{2 \pi \hbar}}e^{ipx/\hbar}$ show that: $$ \left<x \middle| \hat{p} \middle| \psi \right> = -i\hbar ...
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2answers
86 views

Mathematical Proof of Force

I remembered reading an paper a while ago which tried to prove whether force actually exists, or if it a man made construct to aid our understanding of the world. However, I can't seem to find the ...
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9answers
21k views

How can momentum but not energy be conserved in an inelastic collision?

In inelastic collisions, kinetic energy changes, so the velocities of the objects also change. So how is momentum conserved in inelastic collisions?
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0answers
27 views

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} + ...
4
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3answers
4k views

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
26 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. ...
5
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3answers
101 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 ...
1
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1answer
65 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
50 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
104 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?
3
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3answers
7k views

Conservation of momentum but not kinetic energy in inelastic collisions

In inelastic collisions, the kinetic energy of the system is not conserved but the momentum is. Kinetic energy is: $0.5 \times \text{mass} \times \text{velocity}^2$. Momentum is: ...
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0answers
42 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 = ...
1
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2answers
39 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 ...
2
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1answer
48 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 ...
2
votes
3answers
457 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 ...
-1
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1answer
49 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
49 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 ...
1
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6answers
768 views

Why is momentum conserved (or rather what makes an object carry on moving infinitely)?

I know this is an incredibly simple question, but I am trying to find a very simple explanation to this other than the simple logic that energy is conserved when two items impact and bounce off each ...
2
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0answers
121 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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0answers
20 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 ...
0
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0answers
33 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 ...
3
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2answers
621 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 ...
1
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1answer
158 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
59 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 ...