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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Force of brake pads on a wheel

I have a hollow-cylinder wheel model, braked with brake pads located at a distance d of the wheel's center axis. The brake pads have a contact area S. They are also forced towards the wheel with a ...
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Cosmic strings and conservation of momentum

I've been told that cosmic strings useful don't give of gravity. I've been told that if an objects passive and active gravitational masses differ then this will result in a violation of conservation ...
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Experiment of liquid filled rod doing oscillation is called?

I know this is a weird question but I can't find any reference to this experiment, I have Google a lot and now I need the experts help. There is a physics experiment where a liquid filled rod is ...
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MOMENTUM With Compressed massless Spring [closed]

How did they arrive at the answer (b)
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Linear Momentum in General Relativity

My question is, does a particle moving in a straight line at constant velocity through empty space create "frame dragging" that would tend to entrain other bodies in the direction of its ...
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Angle-free two-dimensional inelastic collision formula?

I'm trying to calculate velocities (by components - x, y) of two objects (balls) after inelastic, two-dimensional collision. I've successfully implemented the angle-free formula for elastic two-...
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Can you explain me the definition of wave number as defined in theoretical physics?

Wavenumber, as used in spectroscopy and most chemistry fields, is defined as the number of wavelengths per unit distance. The corresponding formula is $$k=\frac{1}{\lambda}.$$ However, in theoretical ...
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Is linear momentum times distance any meaningful quantity? $\vec{r} \cdot \vec{p}$ or $pr = mvr$ comparing to $\vec{r} \times \vec{p}$

Angular momentum is$$ L=\vec{r} \times\vec{p}$$ I was wondering if the dot product has any meaning: $$ ?= \vec{r} \cdot \vec{p}$$ Does it mean anything? It could also be rewritten like $ rp$ or $\...
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Help with understanding virtual displacement in Lagrangian

I know that these screen shots are not nice but I have a simple question buried in a lot of information My question Why can't we just repeat what they did with equation (7.132) to equation (7.140) ...
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Stationary Schrödinger Equation in Momentum space

Given the time dependent equation: $$\partial_t\,\hat{\psi}(p,t) = \dfrac{p^2}{2\,m}\,\hat{\psi}(p,t) + \hat{\psi}(p,t)\star{\hat{V}(p)}$$ and forcing through some kind of separation: $\hat{\psi}(p,t) ...
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Is light's momentum $0$?

From Paul Dirac's extended mass-energy equivalence equation, we know that, E2 = (mc2)2 + (pc)2 We also know that E = mc2 So, if we write mc instead of ...
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Do you have to leave something behind to go forward? (ie a contained system)

By "behind" I mean a particle outside of the box. ( assume a 1d system)Imagine a closed box. And at the very center of that box, is a device that fires particles 1,2 in opposite direction. ...
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Is it possible to move without throwing or pushing another object or energy?

All kinds of movement occur when a thing throws something out or pushes something back and then the thing moves. Like the car pushes the road back, the rockets throw gases at high speed to move. ...
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Can we convert momentum to coulombs in any simple sense?

Let's suppose hypothetically you throw a specially shaped magnet designed with sliding plates, and when it hits the ground it converts almost all it's momentum, or if need be for more complications, ...
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Motion and momentum of spinning mass

Two college friends get together 50+ years after their physics class and begin to discuss forces, momentum, spinning bodies, and space propulsion. It developed that they had widely differing opinions ...
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Superposition of momentum plane waves for a WF: discrete of continuous?

On the one hand there is a theorem that states that any reasonable wave function $\Psi$ can be written as a superposition of eigenstates of $\hat Q$ (a hermitian operator). So if $\Psi _i$ are the ...
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Why is derivative of Lagrangian with respect to generalized position and velocity equal to this?

I'm currently studying Lagrangian mechanics, and in the process, I've met the following equations in a couple of proofs. $$ \frac{\partial \mathcal{L}}{\partial q_i} = \dot p_i $$ $$ \frac{\partial \...
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Do Einstein field equations only relate local spacetime curvature to local energy-momentum of matter?

Do Einstein field equations only relate local spacetime curvature to local energy-momentum of matter? If so, can we extend Einstein field equations globally relating global spacetime curvature to ...
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Linearity of exponentials VS formalism [closed]

$\hat p = \frac{\hbar}{i} \frac{\partial}{\partial x} $ acting on $e^{\frac{ipx}{\hbar}}$ gives $p e^{\frac{ipx}{\hbar}}.$ By linearity, we have $\hat p \frac{1}{\sqrt{2\pi \hbar}} \int dp \Phi(p)e^{\...
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Casimir Effect derivation for energy

I was looking at the Casimir Effect, and I computed that $ka = n\pi$ where $k$ is the 4-momentum of the field and $a$ is the spacetime point where the scalar field becomes 0. Since $k$ is a 4-vector, ...
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Ball collision with a wall

I understand that when a ball collides with a wall it exerts a force. By Newton's 3rd law, the wall exerts an equal and opposite force back on the ball. As a result in an ideal world, the ball will ...
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Energy to momentum

Is there anyway to convert energy to motion IN SPACE? Let's say a satellite collects electric energy from sun using solar panel. Is it possible to convert it to Linear motion? The only way I know to ...
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How small is $dt$ in this derivation of the kinetic energy of ideal gasses?

I was reading the derivation of the average translational kinetic energy of an ideal gas in Sears and Zemansky's University Physics. This derivation uses a cylinder with height $|v_{x}|dt$ and base ...
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On what ground do you multiply $m$ with $v$ in the momentum equation $p=mv$? [duplicate]

I've read several other posts that says the momentum equation is the definition of momentum, and it has no proof. However, I would like to know what is the experimental observation where the ...
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Problem with work done by normal force

A block of mass $M$ with a semicircular track of radius $R$, rests on a horizontal frictionless surface. A uniform cylinder of radius $r$ and mass $m$ is released from rest at the top point A(see fig)....
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Heisenberg microscope for momentum

In the "Heisenberg microscope", the position of a particle is measured using a photon. The higher the photon energy, the better the precision in measuring the particle location is, but the ...
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How to Derive Potential Momentum?

The only derivation/definition of potential momentum I've seen is using the fact that: $$E^2-p^2c^2=m^2c^4$$ And if you add a potential, you must subtract something else from the momentum called the ...
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What gives particles mass? Also why are particles like photons considered massless when they have energy and momentum? [closed]

Interested in knowing the quantum explanation for what gives particles mass, and why particles like photons are considered massless when they have energy and momentum.
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Moving cart, falling rain, and constant velocity

A cart moves in a horizontal line with constant velocity and rain starts to fall on it with water at a rate of $x kg/s$. The force that must be applied to the cart so that the velocity remains ...
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Effect of Momentum Operator in Position Eigenstate [duplicate]

In Lectures on Quantum Mechanics by Steven Weinberg, section 3.5, he asserts that we can infer $$P_{j} \Phi_{\mathbf{x}}=i \hbar \frac{\partial}{\partial x_{j}} \Phi_{\mathbf{x}}\tag{3.5.11}$$ from ...
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In a seesaw, is it possible to consider either of the ends as the fulcrum?

Clasically, we learn the pivot point of the see saw as the fulcrum, and the moments are calculated as F*D, where D is the distance from the fulcrum. However, is it possible to mentally consider one ...
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Is Conservation of Linear Momentum subservient to conservation of Angular Momentum?

When particles physically interact, they transfer linear momentum and angular momentum between one another via force . When particle P1 exerts force on particle P2, P2 exerts an equal and vectorially ...
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Why isn't momentum conserved here? [duplicate]

Suppose I throw a ball horizontally towards a wall with momentum $\vec p$. Let it collides with the wall and then rebound back towards me with momentum $-\vec p$. Since the wall remains stationary, ...
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In the case of a free wave function, does $|p| = E_k = 0 \mapsto |v|=0$?

This answer indicates that "it is not possible to talk about a particle 'stopping' or being 'stopped' in any meaningful or non-contrived sense" because of the Heisenberg indeterminacy ...
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9 votes
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Why are CFTs not usually studied in momentum space?

Conformal symmetry in QFT has been extremely useful for physics. However, while most of QFT is usually done in momentum space, CFTs are usually studied in position space or in terms of Mellin ...
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When we move do we "borrow" momentum from the earth?

In 2001 Arthur C Clarke wrote: Like a ball on a cosmic pool table, Discovery had bounced off the moving gravitational field of Jupiter, and had gained momentum from the impact... Yet there was no ...
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Can force be defined as the rate of change of four-momentum in General Relativity?

In Newtonian physics, the force acting on a particle is defined as the rate of change of momentum $$F=\frac{dp}{dt}.$$ Also, the force can be defined as the derivative of the potential $$F=-\frac{dV}{...
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Velocity of a boat after getting off of it

A 50 kg person gets off of a 100 kg boat at rest with a velocity of 10 m/s. What will be the velocity of the boat in the opposite direction? Apparently, the momentum of the person and the boat are ...
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The general form of the distribution function and an example

I am reading Landau & Lifshitz's Statistical Physics. On page 12, section 4, the distribution function for a closed system $$\rho=constant\times\delta(E-E_0)\delta(\mathbf P-\mathbf P_0)\delta(\...
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An exercise about angular momentum in "The Mechanical Universe" (Frautschi)

I'm reading through "The Mechanical Universe" and I'm stuck with one exercise in the chapter about angular momentum. It goes like this: A particle of mass $m$ is at the end of a rod of ...
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How to Calculate speed of body required for collision to have specific output

I'm trying to calculate the speed the bolt of an airsoft rifle has to go in order for a bb to have a specific speed. for example: the bb weighs 0.2g and the bolt I have weights 100g, I would like the ...
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When solving problems on linear momentum, when can external forces be neglected?

I was recently solving a problem in which one end of a massless string (in vertical orientation) was tied to a block of mass $2m$ and the other end to a ring of mass $m$, which was free to move along ...
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Elastic Collision: question on Wiki equation

I'm confused over the Wiki equation for an elastic collision. Does anyone know how equations (1) and (2) are formed to result in (3)? I think I'm overlooking some simple algebra. Consider particles ...
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Are there two competing definitions of "inertia"?

The term inertia is often introduced by stating Newton's first law: An object stays at rest or moves with $\vec{v}=const.$, if the resultant force is zero. This feature of masses is called "...
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Is linear momentum of an open system conserved?

My understanding is that a system is a collection of particles. And according to Wikipedia, a closed system is one that does not allow transfer of matter in and out the system. However, my textbook ...
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Intuition for momentum operator in position space

The derivation of the momentum operator in position space. But, several assumptions are usually made that a) we are dealing with the particle in free space or b) that the two representations are ...
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FLRW metric, universe expansion, and the energy-momentum relationship

This is a follow-up to a previous question of mine. I am getting myself confused by some basic things in cosmology, so I hope whoever reading this is patient. The Euclidean FLRW metric is given by $$ ...
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Question about momentum in the FLRW metric

I'm reading through Modern Cosmology by Dodelson and Schmidt 2nd edition, and I am wondering if anyone can comment more on the following part. In Section 2.2, we define the Euclidean FLRW metric by $...
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Two objects of equal mass, collision with friction

If two objects with equal mass collide on a floor that has friction, why is momentum not conserved? I understand that in collisions where one object is initially at rest, the force of friction on both ...
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Why center of mass constant is zero?

The Center of mass frame (CM) is defined in such a way that total momentum in this frame is zero. Thus $\vec P'=\sum (m_i\vec v_i')=\sum \big[m_i(\vec v_i-\vec u)\big] =\vec P - M\vec u=0 \tag{1} $ ...
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