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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How can you find the impact necessary to change the direction of Earth's spin?

If an object the mass of the moon was to hit earth's surface at an angle that would be in the opposite direction of earth's spin, how can you find how much momentum the object needs to cause earth to ...
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What happens to Newton's Third Law during the reflection of a pulse from a free end?

Well, it is known that a pulse gets inverted when it gets reflected from a fixed support while the polarity of the pulse remains same when the incident pulse gets reflected from a free end. However, ...
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44 views

Momentum Twistor variables and non-planar theory

I know that the use of twistur-momentum variables makes manifest the arising of certain poles in scattering amplitudes: if the sum of external momenta $P_I = p_i + p_{i+1} + ... + p_j$ is going on-...
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62 views

Is $E = p^0$ in non-inertial frames?

In special relativity the energy of a particle is the 0-component of the 4-momentum. However, is this still true in non-inertial frames, and in particular in non-freely-falling frames in general ...
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328 views

Portal 2 physics - one moving portal [closed]

A problem is posted and solved so many times on 9gag - till yet no good answer. The problem: you have two portals (one moving and one still standing). The one moving reach a still standing cube: what ...
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60 views

Is my expansion of the state $| x \rangle$ correct? [duplicate]

In my quantum mechanics textbook it says that the relation between the basis $|x\rangle$ and $|p\rangle$ is given by: $\langle p | x \rangle = \Large \frac{e^{-ip x/ \hbar}}{\sqrt{2\pi \hbar}} \, .$ ...
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84 views

Derivative of the action integral [closed]

I need to find the partial derivative of the action $S$ with respect to the generalized coordinate $q(t_f)$ and according to my textbook, it should equal the generalized momentum $p(t_f)$. I have a ...
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66 views

Finding the momentum of a muon in $\pi\to\mu\nu_\mu$ [closed]

The $\pi$-meson's mass is 139.57 MeV, and it decays into a muon and a neutrino. The muon has a mass of 105.4 5MeV, and a proper lifetime of $2.197 \times 10^{-6}$. The neutrino can be treated as ...
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84 views

Equating $\frac{dp}{dt}$ and $F$ at a hydraulic jump?

A journal I am reading about circular hydraulic jumps provides the equation and I quote: $H$= Depth after hydraulic jump, $h$= depth before hydraulic jump, $V$= velocity after hydraulic jump, $v$= ...
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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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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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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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65 views

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 ...
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1answer
84 views

The Egg Smashing Tournament

My cousins and I have a annual egg breaking tournament. First, each of us gets a hard boiled egg. Then, in a bracket tournament style, a pair of us face off in an egg breaking round: one person holds ...
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86 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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68 views

Making An Energy Momentum Plot For A Rashba Model (Using Discretization)

I want to make a plot of the Energy versus the Momentum of the Rashba model, using discrete matrices. First Ill show how I did this for the free particle. Subsequently I will show what goes wrong for ...
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121 views

Momentum of capacitor in a uniform magnetic field

We are observing ideal, charged, parallel plate capacitor placed in uniform magnetic field parallel to plates. Whole system is at rest and isolated (we have forces that hold plates separated, but net ...
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212 views

The relationship between angular and linear momentum

Why is orbital angular momentum not 0 when spin and linear momentum are not collinear? Why can it be 0 when spin and linear momentum are parallel? Like in the example of a scalar field at rest ...
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53 views

Sub-light speeds and momentum conservation law

Let's imagine a boat on a lake. Observer A is sitting on the shore. Observer B is sitting in the boat on the bow. Observer B has a ball attached to the end of a string which he holds in his hand. ...
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317 views

If photons don't have mass, how can they accelerate objects? [duplicate]

As far as I know photons don't have mass but they do have momentum ($p=mv$). Scientists say that if we put a shiny (reflective) shield of large radius in the vacuum of space, then light from sun will ...
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2answers
13k views

How to find the compression of a spring attached to an object [closed]

I am having some trouble figuring out the equation needed to solve this problem. A 3.0-kg block slides along a frictionless tabletop at 8.0 m/s toward a second block (at rest) of mass 4.5 kg. A ...
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kinetic energy in collisions [closed]

We were hoping you could help us understand collision energy. Vehicle $ A $ is driving West at $35\space mph$ and weighs $ 1437 \space kg$. Vehicle $ B $ is driving North at $35\space mph$ and weighs ...
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316 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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681 views

How do waves have momentum?

A question on a practice test I'm taking is as follows: By shaking one end of a stretched string, a single pulse is generated. The traveling pulse carries: A. mass B. energy C. momentum D. ...
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Do particles also have intrinsic linear momentum (linear analogue of spin)?

We know from quantum mechanics that microscopic particles have spin, which is a kind of intrinsic angular momentum. The particle has angular momentum without physically rotating. In a similar way, do ...
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Why is there no relation between kinetic energy and momentum in collision of two bodies? [duplicate]

The statement that baffles me: During most of the collisions, part of the kinetic energy evolve as heat, nevertheless momentum is still conserved. Ok, the statement may be true. But what ...
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203 views

Why is $M\frac{dv}{dt} = v_{rel} .\frac{dm}{dt}$ correct and $(M - dm)\frac{dv}{dt} = v_{rel} .\frac{dm}{dt}$ wrong?

Newton's 2nd law of motion can't be applied for mass-varying systems. Another force, known as Thrust must come to play. It can be measured using law of conservation of linear momentum. $$\text{Thrust} ...
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248 views

Why don't we substitute for $p$ in $E = pc$?

See, the energy of a photon is given out by $E = pc = hv$ why don't we substitute for $p$ in $E ^2= p^2 c^2 + m^2 c^4$ by putting $p = \gamma mv$ and then get a value for $m$ (which will be $0$ for a ...
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Can momentum be conserved in a perfectly elastic collision?

I am taking for granted that when we say that something is conserved it is understood 'in its full integrity'. Energy is represented by a scalar J, and is conserved in elastic collision. momentum ...
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Why is scattering vector $\vec{q}$ called vector of 'momentum transfer'?

In the world of scattering the angle at which you detect the scattered radiation is known as $q$, where $$ \vec{q} = \frac{4\pi\eta}{\lambda}\sin(\theta/2) $$ I read in a lot of books that this is ...
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A spinning bullet

I know the rifling in a gun or rifle puts a spin on the bullet along the axis of trajectory. Now I don’t understand exactly why does it make the trajectory more stable and allow for greater travel?
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Momentum operator representation

If $\hat{p}$ acts on position eigenstate, it is $$\tag{1}\hat{p}\left|x\right\rangle=+i\hbar\frac{\partial }{\partial x}\left|x\right\rangle .$$ But in general $$\tag{2}\hat{p} = -i\hbar \frac{\...
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408 views

Why is the momentum of a particle $\gamma mv$?

I am very new to relativity, and as I was going through Resnick & Halliday, I saw that momentum was no longer $mv$, rather $\gamma mv$. This was the proof: $$p = mv = m \frac{\delta x}{\delta ...
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586 views

Momentum, Impulse and Newton's Second Law of Motion

Newton formulated his Second Law as such: $$\sum{\vec{F}} = \frac{\delta \vec{p}}{\delta t}$$ and of course, $\vec{p} = m \vec{v}$. Why is it that if the net force $\sum \vec{F}$ is constant (which ...
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Does momentum increase with out of phase photons?

This paper speculates that the EM drive produces thrust with out of phase photons: http://scitation.aip.org/content/aip/journal/adva/6/6/10.1063/1.4953807 My question is this, do out of phase ...
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When a bomb explodes, does it momentum remain same?

If, from an aircraft, a bomb is thrown to an object placed at ground and bomb explodes before it hits the object, i.e if it explodes in the middle of its path, does it momentum remain same? I knew ...
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5answers
147 views

If “For every action there is an equal and opposite reaction”, why does a gun not injure the shooter's hand?

If "For every action there is an equal and opposite reaction", why does a gun not injure the shooter's hand? Would not the "equal and opposite reaction" of firing a bullet be the kickback the shooter ...
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2answers
459 views

Photons have no mass. So, why does $E = pc$ hold? [duplicate]

It's a somewhat theoretical question. In special relativity, The energy of a photon is given by $E = pc$. But, my argument is that, since photons have no mass, how can they have a momentum $p$? The ...
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2answers
105 views

Optimal curve for a marble drop [closed]

I'm doing a marble roller coaster project for my physics class at school. The first part of our roller coaster involves a marble falling downwards into a curve that will drop then go up 12 inches. ...
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5answers
202 views

What is the definition of linear momentum?

Every where and book I search I get that the definition of linear momentum is the amount of speed (quantity of motion) contained in it or simply it is mass $\times$velocity? So, what is an ...
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1answer
266 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
746 views

What does the force mean in $dp/dt$ form?

One was to write Newton's laws is: $$F=\frac{dp}{dt}.$$ I don't understand what is the force there. I believe that $F$ is the net external force on the system. So supposedly I have a mass which is ...
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4answers
330 views

Question about dot product of four vectors?

I am looking back over some old notes and see that I have written $\bar{p}=\left(\overset{E}{\vec p}\right)$ and $\bar{x}=\left(\overset{t}{\vec x}\right)$ (using Plank units) And then $\bar{p} \...
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434 views

Is linear momentum conserved in an isolated system always?

Consider a isolated system of particles. If no external force acts on this system, the momentum of the system should remain constant. If the isolated system of particles represents living creature....
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623 views

Question relating to energy transfer in between two particles

How would I go about calculating the maximum kinetic energy transferred from one particle when it forms an elastic collision with another particle. For example, if I had two billiards one with mass m ...
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Application of Newton's third law

By newton's third law , we know that every action has an equal and opposite reaction .We move left when we push slightly on the right and move forward as we push backward. But how do rockets, ...
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7k views

When is energy conserved in a collision and not momentum?

Consider the following example: A bullet of mass 45g is fired at a speed of 220 m/s into a 5.0 kg sandbag hanging from a string from the ceiling. The sandbag absorbs the bullet and begins to swing....
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239 views

QED: basic process which violates energy-momentum conservation

Why is the following process not possible? My book says it's because it violate energy and momentum conservation. Can someone explain me explicitly why? Why couldn't an energetic electron not ...
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2answers
451 views

Connection between momentum and energy

What is the connection between momentum and energy? Which of the answers is the correct? A particle can have zero momentum but energy. A particle can have zero energy but momentum. ...
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1answer
127 views

On a frictionless table, what is the angle at which a cue ball should hit another ball for it to go in?

Assume there are 2 balls on the table. Cue ball and another ball. What is the angle at which the cue ball should hit another ball for it to go in?