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Questions tagged [momentum]

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 much more damage is caused after hitting a brick wall at 100mph over 50mph? [duplicate]

How much more damage is caused after hitting a wall at 100mph over 50mph? It is clear that there is just twice as much force generated after hitting a wall at 100mph, since the deacceleration is twice ...
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Moving a (specific)-spacecraft without ejecting mass

The idea is to move this spacecraft without ejecting any mass. The craft has two paddles which we can make spin. So we rev these two paddles up, spinning them in opposite directions and when their ...
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How propulsion in the “HELPING HAND” NetFlix episode works? *SPOILER*

I watched the "HELPING HAND" episode in NexFlix, and the protagonist is drift in space, every second she is more far from her ship, when her tank is out of oxygen, she throw her glove to make a ...
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How does the momentum conservation law hold for gravitational interactions in special relativity with Newton's third law being invalid?

Griffiths' argument that Newton's third law is invalid in special relativity The selected answer to the post above talked about momentum conservation in electrodynamics – Newton's third law ...
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Particle collision, mass non-conservation. (four-momentum, special relativity problem)

A particle with mass M and speed v along the positive x-axis hits a stationary mass m. Two particles, each with mass µ, emerge from the collision, at angles with respect to the x-axis. (a) Write the ...
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Confusion regarding conservation of momentum and conservation of energy [on hold]

When solving the problem under in the two different ways listed, i end up getting different answers. An electron is moving with $v_1=0.87c$ perpendicular to a positron moving with $v_2=0.98c$ in an ...
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Why does energy conservation give me the wrong answer in this inelastic collision problem?

Suppose there are two objects. The first object is stationary and has the next parameters: v1=0 m/s, m1= 4kg. The second object is heading towards the first one with speed v2= 8m/s and mass m2= 60 kg. ...
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Convert inertia formula to excel calculator

I've been looking at this question: "Weight" of moving object in a car collision but to be honest I can't get my head around how to work it out myself (I'm not a physics student!). What ...
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4answers
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How can momentum but not energy be conserved in a collision, if objects keep moving in the same direction?

For example, there are 2 objects with the same mass, moving in the same direction, one with speed 5 m/s and the other 1 m/s. They collide and keep moving in the same direction, now one with speed 4 m/...
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Pellet collides with hanging block, what is the velocity of pellet? Two different methods of answering with different answers which is correct? [on hold]

There are two different methods to solve this question and both of them make sense to me yet they give different answers: Method 1: The energy of the system at the start and at the end must be the ...
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1answer
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Conserved quantities in ballistic pendulum

A bullet of mass $m$ hits a sandbag of mass $M$ that is suspended from the ceiling by a massless rope. At the moment the bullet hits it has a horizontal velocity $v$, and it gets stuck in the sandbag. ...
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Centre of mass and conservation of momentum [closed]

A bullet of mass $50$ g moving with velocity $600$ $m$ $s^{-1}$ hits a block of mass $1$ kg placed on a rough ground and comes out of the block with a velocity of $400$ $m$ $s^{-1}$. The coefficient ...
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1answer
35 views

How can a hammer of very less weight apply a large force when hit from a height?

Consider a hammer of weight 20N . When it is at on the table the only force it can apply on the table is it's weight. so it applies 20N when kept on the table . But when it is raised and dropped, the ...
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3answers
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Is vector sum of momentum constant if there is friction?

In a head-on collision of 2 objects with the same mass and speed in opposite directions, the vector sum of momenta before and after the collision stays the same and is equal to zero. I've read that if ...
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Question regarding energy and momentum conservation [closed]

I have a problem in mechanics. On the wedge and block only the gravitational force ($mg$) is exerted (and there is no friction in this system). What is asked in the question is the final velocities ...
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3answers
60 views

How does conservation of momentum work here? [closed]

Imagine 2 oppositely charged particles attracted to each other, say an electron and a nucleus, assume their force of attraction is very large compared with any external applied force we're about to ...
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How did the bike driver reduce his momentum?

Consider the following GIF The bike driver gained relatively large momentum from the ride. My question is, how the flip he made helped him reducing his momentum and landing safely?
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2answers
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Special relativity - mass energy and momentum [closed]

I've just started studying relativity and I think I have just about gotten my head around time dilation and length contraction and I have now been able to derive the two equations $$t=\frac{t_0}{\...
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1answer
53 views

What is $mass \cdot jerk$, or yank?

So if Momentum is $m \cdot v$, Force is $m \cdot a$, and Jerk is $\frac{\Delta a}{\Delta t}$ what is $m \cdot j$? I've read that the name for this is yank, but I'm curious as to what it is ...
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1answer
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Conservation of quasimomentum and added-mass: help needed to understand a derivation in G. Falkovich's fluid mechanics book

Context Starting at page 30 of Falkovich's Fluid Mechanics, there is an interesting discussion about the relationship between the conservation of quasimomentum in an isentropic ideal fluid and the ...
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1answer
41 views

Elastic collision between a circle and a point [duplicate]

In a 2D environment, I have a circle with velocity v, a stationary point (infinite mass), and I am trying to calculate the velocity of the circle after a perfectly elastic collision with the point. ...
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43 views

Four momentum squared and collisions

So, I am not asking is the square of four-momentum of a particle an invariant to Lorentz trasnformations, but rather,is it invariant in dynamic situations? It seems to me that this also has to hold. ...
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1answer
32 views

Solid state physics, meaning of momentum notated G in photon-phonon collision?

I have a problem in momentum conservation in the scattering of photon with phonon.In the process the momentum conservation is written as k'=k+G+q.where k'=wave vector of scattered photon,k=wave vector ...
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Restriction for the rule: change in momentum=force x time?

I did a prac about impulse and momentum. F∆t=m∆v indicates that the change in momentum should same as the area under force time graph. I ran a experiment where a cart is released on a slope and hit ...
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1answer
31 views

Calculating velocities of 2 squares after collision

If I have 2 squares each with random velocities, and both with the same mass, would it be possible to calculate their velocities after a collision, and how would I do so?
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Total momentum of multiparticle eigenstates of discrete translation operator

I will try to frame my question using the transverse field Ising model in the low spin-coupling limit as motivation. I'll begin by discussing a case I believe I understand, that of eigenstates of ...
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1answer
56 views

Which formula for elastic collision?

When researching a bit about elastic collisions, I've been confused about how many different ways there are to do this. I've seen multiple methods, some of which are extremely complex (using sine and ...
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0answers
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Showing conservation of momentum for arbitrary pendulum trajectories

Consider an isolated system of a pendulum driven by a motor, initially at rest. Conservation of momentum and angular momentum ordains that the center of mass and orientation cannot change in the ...
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1answer
33 views

Which is more important for a force of the impact mass or velocity? [duplicate]

If a 2 times bigger mass collides with an object in which a 2 times smaller mass collides but with 2 times the speed which one will produce more force on the object? The object is stationary before ...
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3answers
67 views

How do rocket engines work in EMPTY space? [duplicate]

So I just read this and it didn't make any sense It is not necessary for the rocket exhaust to push against anything EXCEPT THE SHIP ITSELF. You see, when the combustion of fuel takes place ...
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2answers
35 views

Conserved and invariant quantities in a nuclear reaction

For a reaction $$a+X\rightarrow Y+b$$ in the book, Introductory Nuclear Physics by Krane, he has written that the total energy and the total momentum both in RHS and LHS remains conserved in Lab frame,...
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1answer
39 views

Impulse imparted during elastic collision [closed]

Lets say there are two objects a of 1kg and a very heavy block. The light mass has a velocity of $12$ $ ms^{-1}$ towards right. The heavy block has a velocity of $10$ $ ms^{-1}$ towards right. The ...
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3answers
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AP Physics C: Does the release of a spring add momentum to a closed system?

Two carts of equal mass are on a horizontal surface with negligible friction. Cart A is approaching cart B, which is at rest. Attached to cart B is a spring that is initially compressed. At the moment ...
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1answer
35 views

Simple Hamiltonian in curved space-time

Consider the theory of a free massless scalar field with a non-trivial background metric: $$ \mathcal{L} = -\sqrt{-g} \left( \frac{1}{2}\partial_\mu \phi \partial^\mu\phi\right) .$$ (I prefer the '...
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1answer
45 views

Inverse Compton scattering

Given a collision between a high energy cosmic ray proton (~$10^{20}$ eV) and a CMB photon, I want to calculate the final energy of the photon. I know to proceed, I use the following conservation of ...
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conservation of momentum, a block on an inclined plane free to move

A block with mass $m_2$ is placed on a frictionless inclined plane with angle $\theta$ and mass $m_1$ which can freely slide on the floor. My question is relating the the velocities of the two mass. ...
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Applying energy and momentum conservation to the problem of pulling a bent carpet at a constant speed

Consider this system. A long, thin, pliable carpet is laid on a floor. One end of the carpet is bent back and then pulled backwards with constant speed $v$, just above the part of the carpet ...
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2answers
58 views

Proving an identity relating the gamma matrices

I'm looking to prove the following identity: $$k_a \gamma^a \gamma^\nu K_b \gamma^b p_c \gamma^c \gamma_\nu P_d \gamma^d = 4(p\cdot K)(P\cdot k)$$ I tried this many times but always seem to be stuck ...
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2answers
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Why can't a particle interaction $A + B \rightarrow C$ conserve energy and momentum at the same time?

I learned that in the Feynman diagram for electron photon scattering, the intermediate particle represented by the middle solid line must be a virtual particle because it violates energy-momentum ...
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1answer
52 views

Predicting elastic collisions [closed]

I want to predict how a collision will go with physics. Say I have a perfectly elastic 1-dimensional system with a frictionless ground and two objects of masses say $m_1$ and $m_2$ moving with ...
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1answer
96 views

Energy-momentum relation in SR and GR

There is the famous energy-momentum relation $$E^2 - p^2 c^2 = m^2 c^4.$$ I thought it is always valid. "Always" means it is valid in general, so in GR and SR. It is something like an elemantary ...
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1answer
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“Plushness” of a simple mechanical damper?

What heuristics generally define the "plushness" of a mechanical damping or braking system? For example, if a person or some mass is to collide with a fixed wall or the ground (e.g., after a vertical ...
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52 views

Relativistic vs. non-relativistic Error

I want to clarify a concept for myself. Is it correct to use the following formula for finding the error in measured relativistic momentum and non-relativistic momentum: $$\frac {\gamma mv-mv}{\gamma ...
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1answer
39 views

Question about differentiating wrt. momentum in Srednicki chapter 14

I am having a bit of trouble following a simple integral from the book on QFT by Mark Srednicki - free draft can be accessed at http://web.physics.ucsb.edu/~mark/qft.html - and I was hoping you could ...
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2answers
52 views

Calculation of path integral in QFT

I am studing QFT using the text book of Srednicki's. And I am stuck on one of calculations of the integrals in his book. Consider a harmonic oscillator with hamiltonian: We can write the following ...
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1answer
78 views

Accuracy of Heisenberg Uncertainty Principle for estimating ground state energy of particle in potential well

I've understood the assumptions and logic behind the 'proof' that the ground state of a particle in an infinite potential well has a non-zero energy using the Heisenberg Uncertainty Principle. ...
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2answers
76 views

How and why does momentum get transferred? [closed]

Let us imagine an isolated system and there are two objects of whatever masses and size. Let one object move towards the other object with constant velocity. We know, that when they collide, the ...
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1answer
46 views

Momentum uncertainty for a free particle [closed]

Free particle is in state psi k, where k is the wavenumber. Now i am trying to findout uncertainty in its momentum. I know that for free particle position is uncertain (delta x approaches to infinity) ...
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1answer
49 views

Angular Momentum, and its Conservation

I am confused as to how to reconcile the conservation laws of linear and angular momentum. Are they independent (and how can this be, if $L = r$ x $p$?) Does one supersede the other? In short: how are ...
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1answer
29 views

How do I find the direction of impulse vector in a collision?

I know that impulse is the change in linear momentum. Is the direction of impulse always along the change in momentum vector? Please explain.