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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Non-relativistic Kepler orbits

Consider the Newtonian gravitational potential at a distance of Sun: $$\varphi \left ( r \right )~=~-\frac{GM}{r}.$$ I write the classical Lagrangian in spherical coordinates for a planet with mass ...
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4answers
894 views

Find total energy and momentum of an moving electron in a rest frame

I have an electron moving with speed $u'$ in a frame $S'$ moving with speed $v'$ relative to a rest frame $S$. How do I find the total energy and momentum of the electron in the rest frame $S$? I ...
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2answers
218 views

Having Trouble With The Principle Of Conservation Of Momentum For a Multiparticle System

I'am reading John Taylor's Classical Mechanics chapter 1 page 20 where he proves the principle of conservation of momentum which states "If the net external force $F^{ext}$ on an $N$-particle system ...
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583 views

End position of movement factoring in deceleration

Ok I am hoping to apply this answer to a piece of software, but it uses physics to work out the result so I require some help in that department. I will attempt to explain this the best I can. I ...
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242 views

Momentum question?

The problem: In a car crash, a passenger with a mass of 82kg is not wearing a seatbelt. The car is travelling at 45km per hour. What impulse must the car's airbag provide in order to stop the ...
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63 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?
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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 ...
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61 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 ...
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35 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 ...
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2answers
94 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 ...
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96 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 ...
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60 views

How similar is comparison between the principle behind Newton's Cradle versus AC Current moving down a transmission line?

Anyone who has taken high school physics has seen the following assembly. You drop one ball from the left hand side and the ball from the farthest right hand side gets knocked away. This is to ...
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85 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 ...
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1answer
60 views

I need to know how many pounds of force a 3 million pound object has moving at 1 mph [closed]

How many pounds of force does a 3 million pound object have moving at 0.1, 0.5, and 1 mph?
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1answer
100 views

Heisenberg Uncertainty Principle

Question: The uncertainty in position is equal to the uncertainty in momentum. What is the uncertainty in velocity? What I did: I know that the uncertainty in position multiplied by uncertainty in ...
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2answers
105 views

Ballistic Pendulum Demo Problem

I have a question about the following problem: I got the solution $v=\frac{M+m}{m} \sqrt{2gh}$. But my real question is in the following picture: In the above slide, how can you derive ...
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1answer
43 views

Spacecraft momentum question [closed]

Assume you're sitting in your spacecraft. The mass of you and the spacecraft is 2000 kg. You're traveling at 200km/h through the vacuum of space. You have a thruster pointing in the direction you're ...
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2answers
41 views

Difference between momentum calculated from wavefunction and momentum computed by energy formula

Take a quantum well, for example. The energy of an electron is given by: $E_n=k_n n^2$. Moreover, we know that the momentum of an electron with energy $E_n$ is given by: $p=mv=m(2T/m)^{1/2}$, where ...
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49 views

What's wrong with this simple derivation of energy flux in a photon gas?

In a photon gas, we know that pressure, $P$, and energy density, $u$, are related by: $$P=\frac{u}{3}$$ We also know from relativity that the momentum of a photon is $$p=\frac{E}{c}$$ Finally, the ...
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2answers
72 views

Understanding Momentum

I'm trying to learn more about momentum and I'm a little confused. Based on my understanding, in an isolated system, total momentum is conserved in a collision. Today in class the professor went over ...
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1answer
54 views

How would one compute the angle of deflection, in a relativistic collision - underspecified system?

Consider the simplistic case of two identical mass particles colliding elastically with the second particle initially stationary and the first particle travelling with energy $E$. By conservation of ...
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1answer
249 views

Quantum Mechanics - Finding momentum probability density [closed]

everyone. I got a bit stuck on 2(iii), this is supposed to be a easy question, but i don't know how you get the square term? I thought you just do the Fourier transform, but then I got some ...
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85 views

Measurement of Mass and Momentum of a particle simultaneously

In quantum mechanics can the mass and the linear momentum of a particle be measured precisely or do they commute ?
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64 views

Recoil from a shotgun

First of all I'd like to point out that I'm a complete layman when it comes to physics and I apologise if this isn't the correct forum for this. If I were to sit on an office chair on a laminated ...
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47 views

Calculating the Probability Current of a Travelling Wave

Calculate the probability current density vector $\vec{j}$ for the wave function : $$\psi = Ae^{-i(wt-kx)}.$$ From my very poor and beginner's understanding of probability density current it is : ...
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28 views

Do I need the exact velocity when experimenting with sliding coins?

I'm doing a home experiment but it's not going very well. I'm pushing coins on a table. I'm taking the time for how long it takes coin A to hit coin B and then I divide it by the time between them ...
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189 views

What is the reason behind why a quantum particle cannot be at rest?

So I've seen different reasonings for this; which is correct, or are they both corollaries of each other? 1) For a particle to be at rest, we would know its momentum and therefore by Heisenberg's ...
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103 views

Relative Velocities and Conservation of Kinetic Energy

An object of mass m moves with velocity $v$ towards a stationary object of same mass. Impact is an elastic collision. $v_1$ is the velocity after impact of the mass originally moving $v_2$ is the ...
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2answers
98 views

Equal and opposite reaction - forces coming in pairs - or just common sense?

I've never understood why the conservation of momentum law is taught in schools as "every force has an equal opposite reaction". To me a gun's recoil is self explanatory; the explosion sits ...
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1answer
50 views

Help with momentum question involving vectors/trig [closed]

The problem: A car with a mass of 1200 kg and speed 10 m/s runs into a traffic barrier at an angle of 45 degrees, and is thrown outwards at an angle of 45 degrees relative to the barrier with a speed ...
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1answer
102 views

Classical disintegration of particles, Landau-Lifshitz series on Physics

i read Landau's book recently. In this book p.43 Landau says from (16.1) (16.2) can be write down $T_10$= $p_0^2$/2$m_1$=($M-m_1$)($E_i-E_1i-E_i'$)/$M$ For me, it is hard to understand the factor ...
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2answers
378 views

Energy transfer in elastic collision [duplicate]

In a given reference frame where object 1 (with known mass and velocity) collides elastically with object 2 (with known mass and velocity), can we identify which object loses kinetic energy? Is it ...
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764 views

Friction on an object moving with momentum over a surface

I'm familiar with the equations for friction for a static object and an object moving at steady speed over a surface from high school physics. But we never learned how an object moving only due to ...
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419 views

Harmonic Oscillator Energy to Momentum Expectation Value

If we are given a wave function written in terms of harmonic oscillator energy eigenfunctions how can we determine the maximum possible momentum expectation value? It's a combination of the first two ...
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1answer
284 views

Muon 3 Body Decay

I'm trying to calculate the maximum energy of the electron in the decay muon >electron + electronantineutrino +muonneutrino in the reference frame of the muon having no kinetic energy. $m_m$=mass of ...
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2answers
822 views

Why does the amount of energy transferred depend on distance rather than time?

The change in energy of an object can be determined by the work equation, where work is the change in energy: $$ W = F \cdot d $$ I conceptualize the transfer of energy as simply a series of small ...
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1answer
106 views

Storing kinetic energy in bonds

Let's assume a setup with a static linear molecule with three identical atoms connected by bonds and a single atom, identical to the other three, being shot at the molecule. Let's also assume that ...
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1answer
103 views

Question about Cars: Momentum

Car B rests at the bottom of a frictionless inclined plane. In order to travel a height of 0.6m and maintain a speed of 2 m/s at the end of the track it needs to start with 4 m/s. a) If ...
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1answer
221 views

Calculate force of impact on running dog

I'm going to hold a dog protection tournament. I'd like to give the audience information about the force of the impact of the running dog when it hit(attacks) the decoy on a huge screen. I'm a ...
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1answer
972 views

Expectation value of momentum

I'm having a problem with an expectation value that doesn't seem to add up for me. What I know is, that $\psi(\vec{r})$ is a wavefunction for a particle in three dimensions. The Hamiltonian is given ...
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1answer
177 views

Kinetic energy when $F(t)$ graph is given

The very basic problem here is that the point where this graph ends is ambiguous. I mean, what is the corresponding $y$-value for the point whose $x$-valu is $4.5s$? Then I think we should find the ...
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1answer
187 views

Newton's second law?

$F=ma$. A car strikes a wall at 60 mph. Its acceleration is zero at the time. The force of the car against the wall or vice versa is? To look at the car the force is not zero. Please explain.
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90 views

Minimum $E$ of $p\bar{p}$-collision for $q\bar{q}$ pair with mass $m_q$

I am currently working out the energy required to create a particle anti-particle pair from a collision of a proton travelling along the x-direction with an anti-proton which is at rest. The particle ...
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320 views

What is the momentum of these emitted photons?

5 x 10^7 kg of radioactive material emits energy in the form of photons of red light (700 nm). (Note: photons have no mass.) What is the momentum of each photon? ...
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3answers
1k views

Can conservation of momentum related to Newton's first law?

I know this is a scrap of thought, but the first law states that (from Wikipedia): If an object experiences no net force, then its velocity is constant Is it describing the conservation of ...
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1answer
227 views

Analysis of the impulse of 2 colliding carts under the effect of magnetic repulsion

Hi there! I have a question about an experiment that was conducted. It is related to momentum. 2 carts were put on a track on opposite sides. They were then propelled towards one another at ...
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1answer
779 views

What is the linear momentum of an inelastic and elastic collision?

The arrows in the figure represent the linear momentum of two balls before they collide. If the collision is perfectly inelastic, find the linear momentum after the collision. If the collision is ...
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165 views

Would synchronized dancing be a good way to describe entangled atoms to a laymen?

I was talking my professor about entanglement swapping between light and matter and it is briefly described here: You start out with a crystal capable of doing parametric down conversion of incoming ...
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132 views

Really basic mechanics and Galilean Relativity question

Consider two solid objects: A and B. System 1: A <----- B 10m/s System 2: A -----> B 10m/s ...
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141 views

The time for which rear moving block remain in contact with spring in the following situation? [closed]

I'm a physics tutor. I'm stuck up with this question. I've no clue about how to proceed with this question. Can any one help? A 2 Kg block moving with 10 m/s strikes a spring of constant π^2 N/m ...