# Tagged Questions

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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### Fundamental properties of motion

The first paragraph of the Wikipedia article on the angular momentum operator states that In both classical and quantum mechanical systems, angular momentum (together with linear momentum and ...
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### Movement of man and ladder and their center of mass

Suppose there is a massless frictionless pulley. A rope over it carries a mass $M$ and on other side carries a ladder of mass $(M-m)$ and a man on that ladder, of mass $m$. Now the man starts climbing ...
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### Relativistic kinematics of particle decay

Suppose a particle decays to three other particles. The masses of all particles are assumed to be known and we work in the rest frame of the parent particle. So there are 12 parameters for this ...
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### Finding force exerted in an Inelastic Collision

I did a lab today in Physics in which we launched ball from a spring loaded cannon directly into a pendulum that captured the ball, held it, and swung upwards with it (representing a totally inelastic ...
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### momentum conservation question involving a rocket and a spaceship [closed]

With the engines off a space ship is cruising at a velocity of 230m.s It fires a rocket straight ahead at the enememy vessel. The mass of the rocket is 1300kg and the mas of the ship (not including ...
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### Converting angular velocity to linear velocity through friction

A very basic question here; it's related to this one, but not quite the same. If a rotating rigid body (a sphere for the sake of discussion) with mass $m$, radius $r$ and inertial tensor $I$ has ...
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### What is the linear momentum of an EM wave in a medium?

In free space, the linear momentum density of an EM wave is given by the Poynting vector $\vec S$ over the speed of light squared, $\vec g=\frac{\vec S}{c^2}$. In a medium, $S$ is generally not ...
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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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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, 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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### 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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### 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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### Why is momentum conserved in a control volumen?

I know momentum is conserved in an isolated system wherein there in not mass nor energy flow and the net resultant external forces are equaly to zero. My doubt is when I'm studying a (fixed) constrol ...
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### 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. ...
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 ...