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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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Two bodies, one with mass m and the other with very very huge mass (tending to infinity), moving with velocity v and -v (opposite directions) collide

Assumption: The bodies are moving on the same axis, along their center of mass, and the bodies are perfectly spherical. There is no loss of energy after collision. m2 >>> m1 v2 = -v1 Will the bodies ...
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Commutator of $f(\hat{x})$ and $\hat{p}$

I have a homework problem that asks us to determine $\big[\,f(\hat{x}),\hat{p}\big]$ only by use of $$\big[\hat{x}^n,\hat{p}\big] = ni\hbar \hat{x}^{n-1}$$ as well as assuming $f(\hat{x})$ can be ...
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Conservation of momentum if kinetic energy is converted to mass

There is a moving object. Through an unspecified (science fiction) mechanism its kinetic energy is converted to mass and the object comes to rest. The mechanism is fictional but in good scifi it ...
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3D perfectly elastic collision between two points

There is a high probability, I think, that this question is a duplicate of some other question ... but to may knowledge, it hasn't been posed in this exact manner: Assume we have 2 points, $P_1$ and $...
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Question about solution to Morin Leaky Bucket problem

In Morin’s Mechanics book, a problem called leaky bucket goes: 5.17. Leaky bucket ** At $t = 0$, a massless bucket contains a mass $M$ of sand. It is connected to a wall by a massless spring with ...
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Simple recoil calcuation: Difference in answer from Momentum vs Energy approach

I think I am completely misunderstanding a concept here. Depending on my approach, I get two different answers, which I shouldnt (as I understand)/ The Setup: A ...
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What direction would this photon go?

Say a matter-antimatter pair of particles annihilates linearly (from a reference frame where the sum of the momenta is zero) to create a photon. Wouldn't momentum conservation prevent the photon from ...
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Heisenberg uncertainty principle meaning and using it to determine approximate sizes confusion

I was always under the impression that the Heisenberg uncertainty principle tells us to what precision we can make a momentum measurement given a position one. However, when doing quantum mechanics ...
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Momentum of projectile in the centre of mass frame in a two body system

The COM frame is defined as the coordinate system in which the centre of mass of the system is at rest. In the case of a 2-body system the center of mass coordinate is: $$\vec{R_{CM}} = \frac{M_A \...
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Solving elastic 2-dimensional collision problems where the resultant angle is not provided [closed]

Having real trouble with the question below that is part of my engineering course. I've sussed question 6 but making no headway with Q7. A ball of mass 0.2 kg collides elastically with a heavier ball ...
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What's the point of momentum? [closed]

I know this may sound like a strange question but I have always wondered what exactly the mathematical point of momentum is. I have understood energy to be an attempt to reformulate Newton's 2nd law ...
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Total energy, total momentum of two different balls [closed]

I have a ball out of plasticine thrown horizontally with speed $v$ on a steel ball that is attached from top to the ground on flexible straps in the middle (see picture). The mass of plasticine is $m$,...
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Problem to solve momentum (Classic Newton mechanics) [closed]

First of all, I want to apologize for my English I'll try to explain my self clearly. I want to share one question about Newtonian mechanics from my homework, of which I can't understand the logic ...
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Still Confused about Linear Momentum in a Circle

A point mass with mass $m$, distance $r$ from circle and constant tangential velocity and constant angular velocity is swung around a circle. ($p$ is linear momentum) Angular momentum is radius x ...
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Conservation of Momentum within Conservation of Angular Momentum

$L=r \times p$ where $L$ is angular momentum, $r$ is radius and $p$ is tangential linear momentum. Using a generic example of a skater spinning on ice with no friction while being stationary, ...
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Why can a particle decay into two photons but not one?

I recently read an old physics news about the Higgs boson where it was observed to decay into 2 photons and I was wondering why it wouldn't have decayed into a single photon with the combined energy ...
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Magnitude of mass polarization

When solving many-body atomic Hamiltonians, (such as Helium), there exists a term in the Hamiltonian which looks something like: $$ \begin{equation} \frac{1}{M}\vec{\nabla}_i\cdot\vec{\nabla}_i \end{...
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What makes a collision 2D?

In my mechanics course, we've covered 1D and 2D collisions. However, what physical factors determine whether a collision will be 1 or 2 dimensional? To make the question more precise, consider a ...
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Which specific law of physics is broken by the “double jump”? [closed]

So, there's a cliche in computer gaming known as a "double jump". This can be described as: A character jumps from a solid surface, and then is able to extend their jump by carrying out the jump ...
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What are the allowed wavenumbers in the finite size system?

Usually, we introduce wavenumber $\textbf{q}$ by Fourier transform, for example, an operator $A_{\textbf{q}}=1/\sqrt{N}*\sum_{i}e^{i \textbf{q}\cdot \textbf{r}_{i}}A_{i}$, where $N$ is number of sites,...
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Can I theoretically completely convert the kinetic energy of a bullet to rotational energy of a disc, when the bullet hits it tangentially?

Will the kinetic energy of bullet be converted to rotational energy of disc(assume the bullet gets stuck to the disc). Let me assume that disc is mounted on a car standing on a frictionless surface. ...
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How does this magnetic toy work?

A nephew of mine has this toy called "Gravitrax". It is basically a track building kit - you can build various tracks for iron balls which go through the track thanks to gravity. However, there is ...
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Momentum in the rest frame

I am a little confused over the terminology of the rest frame. Say a photon collides with a massive particle then from the rest frame of the massive particle would it be the photon or the particle ...
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In a masses inelastic collision, K energy convert to heat but velocity remain same. we receive heat without losing velocity? how?

I have read the answer about the energy and momentum conservation question but I have a great question now! Imagine we have two masses which one of them is at rest and another move and collide to the ...
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Which are the underlying Lie group and algebra related to the translation invariance in field theories?

I'm new to Physics SE. I've seen a lot of interesting questions and answers, and thought it will be very useful to participate a little. I'm currently stuck in a, probably, very simple matter, ...
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Is outside force needed to conserve angular momentum?

Franklin Hu has a good experiment here. Franklin uses the correct formula for angular momentum; and he realizes that the Linear or Newtonian velocity must double if angular momentum is to be ...
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Trace over configuration basis

Let us take a many-body quantum system, whose phases in the configuration basis are labeled by $\mathbf {\hat q}=(q_1,\cdots, q_N)$ and momenta $\mathbf {\hat p}=\left(-i\frac{\partial}{\partial \hat ...
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Does Oberth Effect work in free space or it works only in gravitational field (gravitational well)?

From Wikipedia article about Oberth Effect it is not clear if it works in free space (no gravity, no air, etc). For example, in very far space between 2 galaxies, gravity is almost zero. There are 2 ...
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Transfer of momentum from a wave to it’s waveguide based on theta and the inertia of the wave?

Bit of a complicated one - I've been trying to understand how the energy in a wave dissipates or transfers it's energy to the walls of a wave guide as the angle of the wave guide changes. As the ...
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2answers
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How does 2 spinning disks and a stationary ball interact in a collision?

Suppose there are two congruent disks that are both rotating on its center axis but in opposite directions. A stationary ball is fed in between the two disks, where it collides with both disks and ...
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Is There a Limit to The Energy-density Confined to a Black-hole Structure, Consisting of a Schwarzschild Sphere? [closed]

Imagine two particles repeatedly colliding with increasing momentum,like a dense collection of particles confined within the envelope of a collapsing star that's close to becoming a black hole. Is ...
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General proof of $m^2 = E^2 - p^2$

All proofs I know of, starting from Einstein's famous 1905 article, only deal with special cases (actually, yes, I know that formula like $E=mc^2$ were known before Einstein in the context of ...
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Collision betweeen two rotating disks and a ball, radius cancels out? [closed]

Suppose there are two congruent disks rotating at its center in opposite directions at the same magnitude, and a ball is put into the spacing between the two disks to be shot out. The ball was a rest ...
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Why does shooting a handgun accelerate a bullet to deadly speed without injuring the gun user's hand?

Momentum is defined by the product of mass and velocity. Now a projectile out of a gun has to have high velocity to penetrate a human body, as its mass isn't significant. But to reach this velocity, ...
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When atom absorb energy, how does the nucleon know? [duplicate]

Imagine a photon hits a hydrogen ion and is absorbed, how do the nucleon of the hydrogen knows and start to accelerate? Any difference between this and hydrogen ion in a electric field?
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Where do I go from here to show that linear momentum is conserved under all instances of translation symmetry?

I've worked through a simple derivation of symmetries implying conservation laws from an invariant Lagrangian. Namely a quantity $Q$ is conserved in the equation below, where $i$ is a degree of ...
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Generic Differential Equation for Oberth Effect

In Wikipedia article about Oberth Effect there is an example of calculation in a simple case when the differential equation is not required. In non-English versions of this article (for example, ...
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Where did mess up while calculating the expected value of the momentum squared?

I have the correct answer except with a negative sign. The wave function is given as, $$\Phi=A\exp\left[-a\left(\frac{mx^2}{\hbar} + it\right)\right]$$ By squaring the momentum quantity, I found ...
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Does conservation of angular momentum break conservation of momentum?

Say we have a spinning ring of mass $M$, rotating at $W_0$, at a radius $r$ from some pivot point. This ring has massless spokes extending out to a length of $2r$. From this, we can calculate the ...
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I have a thought experiment that defies the uncertainty principle - what's wrong with it?

There's a particle somewhere on the x-axis (this whole thing is 1-dimensional) at x0, which we do not yet know. I fire a photon at it from the origin with a known speed (c) and a known wavelength λ1 ...
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Where does the momentum go when atom absorb a photon? [duplicate]

Imagine an electron around an atom absorbs a photon and becomes excited, it has now jumped to a higher orbital. At this point in time, where does the momentum of the photon goes?
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Calculation of the Impulse of the force

can we calculate impulse of the force acting on a particle for a time interval using using Newton's 2nd Law which is described as Force acting on a particle equals mass of the particle times ...
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Impulse as the time derivative of force

Momentum is the time derivative of force. $F = \frac{dp}{dt}$. But isn't impulse $J=Ft$, $F=\frac{J}{t}$? Can impulse be the time derivative of force too, or is it just defined as an integral?
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Does the wave function carry energy and momenta?

I was reading a forum regarding the reality of the wave function, and one user (who apparently works in quantum foundations) stated that "the wavefunction carries no energy or momenta and this ...
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Action of momentum operator on wavefunction in momentum space

In a previous question How to get the position operator in the momentum representation from knowing the momentum operator in the position representation? it was mentioned that $$\begin{align} \...
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Momentum of molecule after absorption of photon

I came across this question in text book and was not sure how to solve it. When momentum is conserved then $$ m_{H_2} \Delta v_{H_2} = -m_{Photon} \Delta v_{Photon} $$ But then photon has a zero ...
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Violating Newtons First Law!

Suppose you are inside a very large empty box in deep space , floating ( i.e not touching the box from anywhere initially).The box is at complete rest. Now you push the box forward from inside. Now ...
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Are angular and linear momentums dependant on each other?

Or more specificly: Is it possible to have a system where angular momentum is conserved,but not linear?
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How can electron have known momentum?

In many exercises it is given that electron has a know speed, e.g. 5*10^3 m/s. But how is this possible when we deal with Heisenberg principle?
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If force is given by the negative gradient of potential, what gradient gives us a momentum field?

Does this question make sense physically? If we wanted to know the momentum of some particle moving through a field at some given point (assuming it's conservative), what function's gradient would ...