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.

learn more… | top users | synonyms

0
votes
3answers
73 views

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?
1
vote
1answer
87 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 ...
2
votes
1answer
48 views

How are momentum and position operators dependent on the chosen inertial frame?

How are momentum and position operators in quantum mechanics dependent on the chosen inertial frame of reference?
2
votes
2answers
78 views

Does constant velocity of center of mass imply linear momentum is conserved?

I know that if momentum is conserved for a system, you can derive that the velocity of the center of mass of that system is constant. I was wondering if the second condition also implies the first: if ...
1
vote
1answer
63 views

Relationship between momentum and energy for rocket-powered aircraft

I'm trying to write a problem for my students in an algebra-based physics class. We have a flight simulator and I've got a glider in the simulator that has a couple rocket boosters attached. Students ...
0
votes
2answers
80 views

Interpretation of the four-vector $k$ in scalar QFT

I'm studying the canonical quantization of the Klein-Gordon real scalar quantum field theory, given by the classical Lagrangian density $$\mathscr L = {1\over ...
-1
votes
3answers
168 views

Is the kinetic energy of an electron always $1.6 \cdot 10^{-19}~\text{J}$?

I was doing the following problem. For $d$. They bizarrely added an extra step. They calculated the momentum of the electron to find the De Broglie wavelength using the kinetic energy relationship. ...
0
votes
2answers
97 views

Playing catch in space?

I was thinking about this on the bus (exam this week :P). What would happen if two people played catch in outer space? Lets say person A and B are in space at rest, A throws a mass m at B, what ...
7
votes
7answers
3k views

Ball flying towards me or me flying towards ball

Suppose a ball is flying towards me at a speed of 10m/s and that, on impact, I feel "x" amount of pain. If, instead, it was me flying towards the ball at the same speed, with all other conditions ...
0
votes
1answer
91 views

Origin of momentum. Noether's theorem

My professor talked about Noether's theorem and how translation is the origin of momentum conservation. But why is it not velocity that is conserved but mass times velocity. And on the same note why ...
0
votes
0answers
13 views

Low-momentum behaviour of a short-range potential

I've read the follow sentence in a journal article (arXiv preprint) which constitutes part of my coursework. As discussed in the previous section, the low-momentum behavior of any short-range ...
2
votes
1answer
286 views

Total Momentum From a Standing Electromagnetic Wave

How does one show the momentum imparted to a perfect conducting resonance cavity (boundary) of any shape by a classical standing electromagnetic wave inside is zero? It should be by conservation of ...
0
votes
2answers
99 views

Why do these two derivations of the formula for impulse contradict?

I was trying to derive $I = F\Delta T$ from $p = mv$. First I converted $v = at$: $$\begin{align} p &= mv \\ p &= m \times at \\ p &= ma \times t \\ p &= Ft \\ \Delta p &= F ...
0
votes
4answers
173 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. ...
0
votes
0answers
24 views

How does momentum effect velocity? [duplicate]

I'm in AP physics and we haven't even covered this yet so sorry if my question seems strange... I know $p = m v$, However right now I am writing a 2D physics engine and I am having some trouble with ...
0
votes
1answer
313 views

What is the time dependent expectation value for momentum

For an electron in an equal linear superposition of two eigenstates in a one-dimensional infinitely deep square potential well of width L, how do you isolate the time dependent part of the expectation ...
2
votes
2answers
213 views

A student of mine wants to build a Gyroscopic Inertial Thruster

I work as a tutor with students in their first three semesters. One of my more eager students is obsessed with this thing: A Gyroscopic Inertial Thruster Here is a Wikipedia Article on that. I was ...
1
vote
3answers
552 views

Impulse and force in a karate chop, can we maximize force minimizing the time of impact?

I have heard that we can maximize the force by minimizing the time of impact in a punch. If that is true, the paradox is if we increase the time of contact with the same amount of force still we ...
1
vote
0answers
75 views

Throwing a baseball against the back of a car [closed]

We, some fellow students and I, are trying to figure out what to. But we simply do not know which formula to use at question a. We found the following: $\Delta P=(2mv_{ball,i}M)/(m+M)$ Could someone ...
0
votes
2answers
81 views

Mathematical subtlety in a physics problem

A wagon of initial mass $M$ is moving with an initial velocity $V_o$. Now sand starts accumulating on this wagon, increasing the total mass from $M$ to $M+s$. We have to find the final velocity of ...
0
votes
1answer
67 views

Why does accounting for direction give the wrong result for a bouncing ball?

This a conservation of momentum problem, gathered from an old textbook. I thought it would be simple, but I seem to be goofing up somewhere in my basic conceptual understanding. PROBLEM: A 1.0 kg ...
1
vote
3answers
112 views

Work done after explosion of chemical bomb

Question: Suppose, a chemical bomb is stationary. Then it explodes into n parts in all direction. What is work done? My Efforts The bomb is stationary, so momentum and and kinetic energy is 0. ...
0
votes
1answer
165 views

Can Hooke's law go along with Newton's 3rd law & how is momentum conserved in case of spring?

Let there be a spring in its relaxed state- that is neither compressed nor extended. One end is fixed on the left and other end is free in the right. Now, I stretch the spring by pulling the free-end ...
1
vote
2answers
539 views

Question of Conservation of Momentum

I think I understand the concept of conservation of momentum in an inelastic collision. However, what if two objects, one being more massive than the other, started out together and then separated ...
0
votes
2answers
1k views

Elastic Collision And Momentum

The question I am working on is, "Two blocks are free to slide along the friction-less wooden track shown below. The block of mass $m_1 = 4.98~kg$ is released from the position shown, at height $h = ...
0
votes
0answers
22 views

Calculate the distance travelled by hot air before it looses its momentum

Hot air is released from a nozzle with certain velocity into the atmosphere. As the stream travels, it looses momentum and after some time mixes with air.How can we calculate the distance the stream ...
0
votes
2answers
137 views

Penetration of a bullet into a wall

In which of the following situation will the bullet of the pistol penetrate deeper in the wall? Why? Alternatively will the penetration in both cases be the same? Can you explain the concepts behind ...
0
votes
3answers
395 views

Quantum explanation of Newton's Third Law of Motion

Newton's law states that for every action there is equal and opposite reaction. This law explains how rockets fly in space and accounts for the the majority of the lift action generated by a ...
1
vote
1answer
59 views

Calculating speed of spheres in an asymmetric Newton's cradle?

If we assume there is an asymmetric Newton's cradle with 3 spheres of different masses and the first one is initialized with some certain speed can we calculate the outcome speed of the all the three ...
0
votes
1answer
102 views

Conservation of momentum 1-D problem: track is not level

This problem is a standard kind of 1-D problem. You have 2 equal carts approaching each other and collide on linear air track. BUT the person setting up the track did NOT do a good job at making the ...
4
votes
2answers
180 views

Where does de Broglie wavelength $\lambda=h/p$ for massive particles come from?

I'm curious where the de Broglie relation $p=\frac{h}{\lambda}$ comes from? I know that for light (which has no rest mass), the following is true: $E=pc$ and $E=hf$ so, $$pc=hf \Rightarrow ...
21
votes
6answers
12k views

Newton's cradle

Why, when one releases 2 balls in Newton's cradle, two balls on the opposite side bounce out at approximately the same speed as the 1st pair, rather than one ball at higher speed, or 3 balls at lower ...
1
vote
1answer
70 views

What matters more in the strength tester game at a carnival, momentum or force?

A buddy and I were arguing over which matters more in the strength tester game (assuming the same accuracy of the hit): 1) momentum or 2) force. He was arguing that if you swing the hammer directly ...
5
votes
1answer
249 views

Is it possible for photon to run in circle by its own gravity?

I have heard that gravity came from energy and momentum so photon has gravity too. Then there are theory state that photon has energy tied to frequency. So if a photon has very very high frequency ...
2
votes
0answers
47 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 ...
0
votes
4answers
3k views

Will a stone thrown in space move forever?

If I throw a stone on space, in a place where gravity is equal zero, will the stone move forward forever, because no air, so no friction?
5
votes
3answers
945 views

Does the canonical commutation relation fix the form of the momentum operator?

For one dimensional quantum mechanics $$[\hat{x},\hat{p}]=i\hbar $$ Does this fix univocally the form of the $\hat{p}$ operator? My bet is no because $\hat{p}$ actually depends if we are on ...
4
votes
3answers
457 views
1
vote
4answers
99 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} ...
1
vote
2answers
100 views

Momentum conservation in an electromagnetic system?

Suppose that I have two charged particles in the configuration below. Let us assume the following: We apply a constant force $f$ to the the bottom particle so that it has a constant acceleration ...
1
vote
2answers
4k views

What is the relation between kinetic energy and momentum? [closed]

If kinetic energy is doubled, what happens to momentum? Is it also doubled? I've tried working through the formulas for each but keep getting lost. $$KE=\frac{mv^2}{2}$$ $$p=mv$$ so if ...
1
vote
3answers
210 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 ...
3
votes
3answers
183 views

How can the linear momentum can be understood physically?

Currently reading Classical Mechanics by Herbert Goldstein, and I'm trying to understand every concept physically. Speed can be understood physically, as the distance traveled within a certain amount ...
-1
votes
4answers
206 views

Inertia and momentum

Question #1: Why does speed have nothing to do with inertia? Question #2: If a car hits a steel wall and stops, where did the momentum go?
1
vote
1answer
271 views

Negative Change in Momentum

Does negative change in momentum always signify decrease in momentum, or could it also represent a change in direction while conserving magnitude of momentum?
3
votes
4answers
1k views

Why Does Angular Velocity Increase as Radius Decrease?

Suppose a child were to ask you why a tetherball (picture below) seems to speed up as it wraps around the pole. How would you explain this to them? Certainly you wouldn't say something like, "Angular ...
2
votes
3answers
5k views

Why does rubber ball bounce back while iron ball doesn't?

Suppose there are two balls, one of rubber and the other metallic. There are of the same mass and are thrown on a wall with the same velocity. Why does a rubber ball bounce back while a metallic ball ...
1
vote
5answers
1k views

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 ...
2
votes
1answer
52 views

Swinging onto a Hill

I was just watching the movie "Frozen" (not the Disney one), and they were trapped in a ski lift 50 feet off the ground. They had to jump. My question is, when they jump, should they swing on the ...
0
votes
2answers
2k views

Relative Velocity and Momentum

The question is, A $45.5~kg$ girl is standing on a $140~kg$ plank. Both originally at rest on a frozen lake that constitutes a friction-less, flat surface. The girl begins to walk along the plank ...