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 will happen to the center of mass of the human body when a person carries a weight with one hand?

What will happen to the center of mass of the human body when a person carries a weight with one hand a briefcase for example. Wouldn't the center of mass move horizontally towards the side carrying ...
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3answers
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Object falls and hits ground - which forces are involved to change its momentum?

When an object falls and hits the ground - which forces are involved to change its momentum? Should $m\vec{g}$ be taken into account of the forces that were involved in the change of momentum?
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35 views

Why would a photon striking an electron “make both recoil”? [duplicate]

Why would a photon striking an electron "make both recoil" as I read in an answer to another question. If the photon is massless, how can it make an electron change momentum?
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46 views

Pressure, Momentum and Energy: A 'Peculiar' Relation

Is the following possible? $$ P = \frac{p^2}{3E} $$ where $P$ is pressure, $p$ is momentum and $E$ is energy. In what convention is the above relation acceptable? EDIT: The source paper can be found ...
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88 views

What is the relation between a basis transformation and an induced transformation $\psi(\Lambda^{-1} p)$ on the wave function? [closed]

I'm having trouble understanding why is $\psi(\Lambda^{-1}p')$ the correct wave function in the Lorentz transformed frame $p' = \Lambda p$. Suppose the state in frame $O$ is given by $$ ...
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110 views

Doubt regarding stress-energy tensor definition

I'm having some trouble understanding the following definition of the stress energy tensor: $T^{\mu\nu}$ is the flux of four-momentum $p^{\mu}$ across a surface of constant $x^{\nu}$. Here's an ...
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203 views

Elastic Collisions and Relative Velocities

In a 1D elastic collision, it is well-known that the relative velocities of the two objects (before and after the collision) are reversed. What is the extension of this result to 2D or higher? Is ...
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86 views

What am I REALLY doing when I take the Fourier transform of the momentum operator

I was playing around with some equations and found a surprising relationship when I took the fourier transform of the momentum operator Define $\hat P = \frac{\hbar}{i} \partial_x$, then $F(\hat P) = ...
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2answers
164 views

Is momentum conserved in the collision of a ball with a hanging rod?

Suppose we have a situation like A ball of some mass $m$ with some velocity collides with rod hinged at point $A$. Is momentum conserved in this situation? I know that hinge will give impulsive ...
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102 views

What is “momentum density” and why it important to QFT?

I am reading Quantum Field Theory for the Gifted Amateur. On page 98, they provide a summary of a basic canonical quantization procedure: Step I: Write down a classical Lagrangian density in ...
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1answer
91 views

Thrust and specific impulse of a rocket

A picture of the question this originates from is given below. Note: I normally work with SI units and am mostly unfamiliar with those used here. The specific impulse of a rocket can be calculated ...
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45 views

Possible Lack of Invariance in Two Inertial Reference Frame Scenarios

Two objects are connected together via a ideal spring which is compressed initially. Then the spring is released, the two balls are no longer connected and go their separate ways. Assume no other ...
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42 views

Rolling on a frictionless pond

I have a doubt about one of the questions in my textbook. Q- You are standing with your bag in your hands in the middle of a friction-less pond. How can you come out of the ice? There is of ...
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1answer
48 views

Transformation of $q_k$ and $p_k$ from invariance of Hamiltonian

This is a step in Nakahara's Geometry, Topology and Physics, 2nd edition, 2003, on pages 7-8: Given that $q_k ' = q_k +\epsilon f_k(q)$, we have that $$\Lambda_{ij} = \frac{\partial q_i'}{\partial ...
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2answers
52 views

How can solar sails be maneuvered?

Photons from the Sun are always radial to it and hence solar sails can only travel radially. Any techniques related to maneuvering?
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76 views

How much energy and momentum is carried by a mechanical wave?

Let's assume there is a wave spreading across in a substance. Described by some partial differential equation. How can I calculate the momentum and energy carried by this wave? Or maybe the energy and ...
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0answers
97 views

Two cars colliding with different speeds

If I have one car travelling at 30 m/s and another travelling at 10 m/s both weighing the same amount, why is it that the car travelling 10 m/s would have more damage? According to Newton's third law ...
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1answer
65 views

Where does energy goes if I jump down from a balloon?

Suppose you are in a hot air balloon of mass $M$ which is floating in the air, and you get out and start climbing down a rope at constant velocity $v$. By conservation of momentum we can show that the ...
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1answer
27 views

When is it appropriate to drop pressure terms when applying conservation of momentum to a fluid?

I'm trying to wrap my head around pressure forces in incompressible, irrotational, invicid flow. Applying conservation of momentum to a control volume gives me \begin{equation} ...
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1answer
85 views

newton's cradle with 2 balls dropped from same height at same time

I'm creating a logo and want it to be as exact as possible. The logo idea is a "Newtonian cradle", but with only 2 balls and on straight steel wire (can't be bend). What would happen if I dropped ...
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1answer
85 views

Sum over momentum states

In our lecture we used quite a couple of times that the sum over momentum states can be approximated by an integral over them. But instead of substituting $\sum_p \rightarrow \int d^3p$, we replaced ...
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1answer
102 views

Conservation of relativistic momentum

I'm trying to prove that momentum, $\rho = m \, v \, \gamma(v)$, is conserved in all frames of reference. I'm having problems with the following situation that I made; momentum is not conserved to ...
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261 views

Is it possible to determine the outcome of any impact knowing only the ratio of masses? [duplicate]

In elastic collisions in 2-D if two balls $A$, $B$ ($m_A = m_B$, $R = 1$) have equal mass we can determine in advance the outcome of the collision. If cue-ball $A$ impacts object-ball $B$ (at rest) ...
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Colliding objects due to gravitational attraction [closed]

If two objects initially at rest, are released and move toward one another due to gravitational attraction, they collide and exchange during collision some amount of impulse, i.e. the object 1 ...
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1answer
206 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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112 views

How does the space shuttle move in a vacuum? [duplicate]

If there is no friction in space, then how does the space shuttle boost? According to Newton's law we move by pushing something; in space, there is vacuum, so how the space shuttle moves without ...
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47 views

a mistake related to variable mass system

I'm having a problem with finding my mistake when trying to find the derivative of the momentum when mass is being ejected in a constant rate. The problem is this - a body in space is burning fuel ...
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1answer
153 views

Find angular velocity of motor

I'm quite bad at this, but I'm trying to change that and I need some assistance. Please bare with me while I attempt to explain what I'm trying to figure out and correct me where I'm wrong. Basically ...
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4answers
139 views

Which is the right explanation for rocket motion? [duplicate]

What actually causes a rocket to move? Is it the pressure in the rocket engine or the amount and velocity of mass that is being ejected out. The reason I am asking is, I found these two explanations ...
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264 views

What is reduced momentum in “A Dynamical Theory of the Electromagnetic Field” by James Clerk Maxwell?

I was reading Maxwell's paper titled [A Dynamical Theory of the Electromagnetic Field][1]. In part 2, section 3 ("Dynamical Illustration of Reduced Momentum"), Maxwell discusses a mechanical ...
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114 views

Dumbbell rotation

What if apply impulse to dumbbell consisting of two masses and massless rod? It is clear for me that left dumbbell will move straight line. But it is not clear for me what kind of motion will have ...
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2answers
53 views

Can momentum be distributed to multiple objects which travel in different vectors?

This is a question about the conservation of momentum: If there were a cluster of billiard balls floating in space and the cluster was struck by one moving ball, the cluster balls would scatter in ...
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Why we can omit some forces while applying linear momentum principle

While applying linear momentum principle, namely that if force is zero linear momentum of the system is constant, in textbook they don't count for $N$ force from $M \to m.$ This force have component ...
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191 views

Question about impulse and momentum

Check this question first please My question is: Why can't I use equations of motion to get the final speed after rebounding?Acceleration not equal to $9.8 \text{m/s}^2$ or the intial after ...
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61 views

Hydrodynamics: Conservation of momentum violated by a fan in a cylinder?

Consider the following situation: there is a hollow cylinder with both caps removed. The cylinder is submerged into water. Inside the cylinder, we place a fan (or pump) that pumps the water through ...
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Can kicking a falling phone save it from shattering?

So I saw this tip but I don't think this is true, it would be that your leg or shoe is more flexible than a hard floor so the momentum change would be slower right?
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26 views

Selection of system in Conservation of momentum

I came across a question in which a cart is moving and having sand. Suddenly the sand valve malfunctioned and the sand starts falling from the cart. So momentum of which system will remain conserved. ...
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1answer
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Diffracted electron - where does it gain additional momentum?

When an electron is diffracted, the momentum after the diffraction has different direction than before. Where does the electron gain this momentum? This is related to this question, but it's ...
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58 views

Is there a general formula to translate from *canonical* to *physical* momentum?

In Peskin and Schroeder, after having derived a conserved tensor $T^{\mu \nu}$ associated with translations in space-time (the stress-energy tensor), it is said that the charges $\int d^3 x T^{0i}$: ...
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1answer
63 views

Obtaining momentum operator $P^\mu$ from Lagrangian and energy-momentum tensor $T^{\mu\nu}$

I am pretty new to quantum field theory. Given the Lagrangian density, $$ \mathcal{L} = \frac{1}{2} ( \partial_\mu \phi ) ( \partial^\mu \phi ) - \frac{1}{2} m^2 \phi^2 $$ and its energy-momentum ...
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402 views

The elusive difference between force and impulse

Impulse is defined as the product of a force $F$ acting for a (short) time $t$, $J = F*t$, and that is very clear. What I find difficult to understand is how a force can exist that doesn't act for a ...
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66 views

What does the relation between mass and energy of a free particle mean?

What does the Hamiltonian for a free particle mean? Does it mean that the kinetic energy of the particle is in reverse relation with mass? $H$ or $E=\hbar^{2}k^{2}/2m$. Or better to ask: what's the ...
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0answers
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Mass of photon, is it possible? [duplicate]

$P=E/C$ In relativistic mechanics a Photon is defined as. $P=hf/C$ Replacing "P" $ mc=hf/C$ $M=h/CT$ What does it mean, did they have mass?
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Conservation of generalized energies in Newtonian and Relativistic Systems

I was considering the following problem: In a closed system, it is assumed that mass, momentum and energy is conserved. If we label the total mass of the System $M(i)$ at a time i, the total Momentum ...
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2answers
179 views

Ball flying towards me - Newton's third law is violated?

I was trying to answer the question of the flying ball here on the basis of Newtons third law and Momentum conservation. Here is what I have tried. Lets take $m_1$ (the ball) hits $m_2$ (the man). ...
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2answers
197 views

Force of a Train

Imagine that there are two trains and the first train is twice as long as the second train. They have the same mass per unit length and they are traveling at exactly the same speed. If the first ...
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1answer
128 views

Total divergence term and corresponding Feynman Diagram

A total divergence term added to the Lagrangian doesn’t affect the action because the integral of a total divergence vanishes. But if one attempts to derive the Feynman rules from the Lagrangian with ...
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Why doesn't an object that collides with one that is at rest just do a 180?

Newton's third law says "for every action, there's an equal and opposite reaction." So why is it, say, that when a pool/billiards ball hits the wall of the table, the ball doesn't just turn around?
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Conservation of momentum when rain pours into a wagon

Suppose a wagon is moving at constant velocity on a friction-less surface, and rain begins to fill the wagon. The net force on the wagon is zero, so momentum is conserved; as the mass of the wagon ...
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1answer
84 views

What is the motion of a balloon with a person climbing on a ladder?

Suppose you had a balloon of mass $M$ in the atmosphere at rest relative to the ground, with a ladder attached reaching towards the ground. A person (of mass less than $M$) begins climbing up the ...