# Tagged Questions

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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[2] []3> A tennis player receives a ball of mass $60\ \mathrm g$ travelling horizontally at $55\ \mathrm{ms}^{-1}$ perpendicular to the net, and returns it at $28\ \mathrm{ms}^{-1}$ in a direction at $... 3answers 102 views ### How does commutation between the hamiltonian and angular momentum operator (squared) imply conservation of Angular momentum? So we are looking at central potentials in QM; The lecturer poses the question, when is$\textbf{L}$conserved? He then considers the commutator of$\hat{H}$and$\hat{L^2}$. We have; $$\hat{H}=-\... 5answers 151 views ### If “For every action there is an equal and opposite reaction”, why does a gun not injure the shooter's hand? If "For every action there is an equal and opposite reaction", why does a gun not injure the shooter's hand? Would not the "equal and opposite reaction" of firing a bullet be the kickback the shooter ... 0answers 36 views ### Assumptions in two balls falling experiment A typical undergraduate physics class problem to demonstrate the effects of conservation of momentum is to have two balls of different mass falling one above the other (the largest mass below) to the ... 1answer 105 views ### What is the origin of the quantum operators for p and E? [closed] It is always stated the quantum operators for p and E are the ones we´re familiar with (the operator for energy, H=i\hbar\frac{\partial}{\partial t} and the momentum operator, \mathbf{p}=-i\hbar\... 0answers 42 views ### Proving Probability Current and Momentum relationship I am trying to show that \mathbf j = \frac{\hbar}{2mi}\left(\Psi^* \mathbf \nabla \Psi - \Psi \mathbf \nabla \Psi^{*} \right) \,, simplifies to \mathbf j = \frac{\mathbf p}{2m} (|\Psi|^2) . I ... 1answer 69 views ### Path of a swinging object on a rope [closed] With my lanyard in hand (weighted by my keys), a gentle swinging motion will put the keys in pendulum motion, swinging back and forth. Pendulum motion is relatively easy to model since it is ... 3answers 179 views ### Momentum operator representation If \hat{p} acts on position eigenstate, it is$$\tag{1}\hat{p}\left|x\right\rangle=+i\hbar\frac{\partial }{\partial x}\left|x\right\rangle .$$But in general$$\tag{2}\hat{p} = -i\hbar \frac{\... 2answers 57 views ### What is relationship between linear and angular momentum? I'm confused about imparting momentum to a sphere, which causes it to rotate without slipping. Does the momentum applied go: only to the rotational motion and the linear is a consequence of the ... 1answer 60 views ### What makes cart with a moving object in it gain speed (no friction anywhere)? I would like to understand what makes the cart below the moving object move. There is no friction between any of the objects. The cart is moving therefore there is force applied to it. The only force ... 0answers 48 views ### The physics of an anvil [closed] I am trying to understand how an anvil might best be made. An anvil requires two essential characteristics: enough hardness to not be deformed under the blows of the hammer, but as little rebound and ... 2answers 148 views ### Conservation of momentum and conservation of energy I had the impression that conservation of momentum is valid only when we consider no loss of kinetic energy into heat etc. My tutor says that even if we have loss of kinetic energy as heat in a ... 3answers 298 views ### How can I solve this quantum mechanical “paradox”? Let a (free) particle move in$[0,a]$with cyclic boundary condition$\psi(0)=\psi(a)$. The solution of the Schrödinger-equation can be put in the form of a plane wave. In this state the standard ... 1answer 83 views ### Particle sliding down frictionless hemisphere on frictionless surface http://web.physics.harvard.edu/uploads/files/undergrad/probweek/prob87.pdf http://web.physics.harvard.edu/uploads/files/undergrad/probweek/sol87.pdf The problem and the suggested solution are in the ... 1answer 53 views ### Calculating post-collision velocity [closed] Consider two persons$p1$and$p2$of the same mass , such that they are together moving rightward with a velocity of$v/2$Now person$p1$pushes$p2$in the left direction. According to newton's ... 0answers 34 views ### Does inelastic collision mean the colliding particles have to necessarily stick? In a case when particle 1 is moving vertically upwards and particle 2 if moving horizontally and they collide perfectly inelastically, why would the particles stick ? Since inelastic collision means ... 2answers 105 views ### When a ball bounces on a wall at an angle, why does the angle of incidence equal angle of reflection? When a ball is thrown to a wall without spin and bounces, supposing it's an elastic collision, why does the angle of incidence equal the angle "reflection" (relatively to the normal), why isn't it any ... 1answer 82 views ### What causes the skidding of the wheels of a moving car? Momentum or inertia? In a moving car, when brakes are applied suddenly the wheels skid.I have these two explanations in my mind and both seems correct to me.1. The momentum of the car must be conserved so the car ... 2answers 66 views ### Why do$\hat{X}$and$\hat{P}$have to correspond to position and momentum? As far as I understand, in QM we treat observables as operators, and the eigenvalues of these operators are the possible values we can measure of the observables. It is usually simpler to work in the ... 1answer 78 views ### Nail clippers and conservation of momentum As I clip my fingernails using nail clippers, pieces of nails sometimes fly off violently, reaching the other end of the room. What's happening in terms of conservation of momentum, how are the pieces ... 1answer 93 views ### conservation of momentum? [closed] At hyperphysics I got this image, with the same description in text as is in this image It says that when a massive particle (say$A$) moving with a velocity collides with an object having a ... 1answer 63 views ### Is$E = p^0$in non-inertial frames? In special relativity the energy of a particle is the 0-component of the 4-momentum. However, is this still true in non-inertial frames, and in particular in non-freely-falling frames in general ... 2answers 63 views ### Why the speed of a cart stays the same, on a frictionless surface, as the sand falls out? [closed] This is a question on my test. I got it right by guessing. Can someone explain it, in terms of equations of momentum. I first thought of the equation of conservation of momentum: $$(m_{sand}+m_{cart})... 1answer 135 views ### Mathematical proof of an electron can't absorb a photon [duplicate] How can we mathematically prove that a free electron can't absorb a photon totally? 0answers 568 views ### Portal 2 physics - one moving portal [closed] A problem is posted and solved so many times on 9gag - till yet no good answer. The problem: you have two portals (one moving and one still standing). The one moving reach a still standing cube: what ... 2answers 167 views ### Multiple conservation of momentum in one [closed] I am in a Grade 12 Physics course and my teacher gave this question for bonus marks, to be solved with only highschool concepts. I suspect that this problem has to do with conservation of momentum, ... 1answer 49 views ### Four momentum in particle physics I have seen four momentum defined in two different ways. One is P =(E,\vec{p}c) and the other is P =(E/c,\vec{p}). Which one is correct and if they both are then why and when is it more convenient ... 3answers 93 views ### How does the uncertainty principle make sense of the fact that momentum for massive particles depends in part on position? The momentum of an object is in part dependent on the change in position meaning the final position minus the initial position. The equation for momentum is$$p=\frac{m \Delta x}{t\sqrt{1-(\Delta x/... 1answer 54 views ### Lorentz covariant completeness relation Let be $$P^\mu |p> = p^\mu |p>$$ i.e.$|p>$is the eigen-vector of the 4-momentum operator. Where does the following Lorentz-covariant completeness relation come from? $$\int d^4p \... 0answers 60 views ### Is my expansion of the state | x \rangle correct? [duplicate] In my quantum mechanics textbook it says that the relation between the basis |x\rangle and |p\rangle is given by: \langle p | x \rangle = \Large \frac{e^{-ip x/ \hbar}}{\sqrt{2\pi \hbar}} \, . ... 1answer 135 views ### two object connected by a spring : momentum/energy topic [closed] Here is my question below: Question Two objects A and B, connected by a light elastic spring with elastic constant 5.0 N/m, are initially at rest. The mass A is 2.0 kg and the mass B is 1.0 kg. ... 1answer 107 views ### mass attached to spring rotates in a horizontal circle [closed] So here is the question I have: A mass of 2kg rotates at 1m/s in a horizontal circle on a table at the end of a spring with an elastic constant of 50N/m. If the original length of the spring is 2 ... 2answers 145 views ### Violation of the Newton's first law of motion in the movement of spacecrafts in the vacuum of space [closed] Suppose a rocket at rest in space where there is a complete vacuum and no force to influence the rocket: In the left we have the rocket with a metal ball in it. After the ball is thrown out of the ... 2answers 147 views ### Did Newton conduct any experiments to find something called momentum, or was he such a great genius that he was able to spot it intuitively? I wanted to go to the depth of the discovery of classical mechanics, instead of just reading, accepting and learning things. Now my question is not a single question, but it can answer many of my ... 2answers 157 views ### Why does Conservation of Momentum fails because of friction? [closed] I did this experiment in my lab. We had two cars with wheel connected by a string. One car has a spring that can be triggered by a switch at the top of the car. (Frictionless surface and light string ... 0answers 16 views ### Relativistic particle decay in ZMF [closed] This is a problem that I feel is harder than I am making it but say you have: A particle at rest of mass M decays into two smaller particles of masses m_1, m_2 where m_1 \neq m_2 and M > (... 0answers 85 views ### Why Velocity is referred as momentum? [closed] In many text books the velocity is referred to as a linear momentum which is being convected. For example the table in the following page My old conception is that the momentum or more precisely ... 0answers 42 views ### Comoving and physical momentum in a Friedmann universe It is most probably a very basic question, but I'm a bit stuck with it. Let us consider a spatially flat Friedmann universe with the usual metric$$ds^2=dt^2-a^2(t)\left(dr^2+r^2d\vartheta^2+r^2\sin^... 3answers 160 views ### Conservation of Momentum Conservation of momentum can be derived during collisions by using Newton's laws of motion. But in other cases, do we simply take it like an axiom ? 2answers 86 views ### Mechanics impulse related A bead of mass 'm' is connected at one end of an inextensible massless string. The other end of the string is fixed to a fixed cylinder. An initial impulse J is given to the bead perpendicular to the ... 2answers 108 views ### Conservation of linear momentum (classical mechanics and special relativity) How did Newton deduce the law of conservation of linear momentum? Can it be derived only by Newton's laws, or does it follow from practical experiments? If the law of conservation of linear momentum ... 1answer 156 views ### How do momentum get transferred? Simple Question , Consider two objects namely$A$and$B$where$B$is stationary and$A$is moving towards$B$with velocity$v$. When the two objects touch each other what does actually happen ... 0answers 44 views ### Gravitons, photons and conservation of momentum How can gravitons be emitted from a mass to cause an attractive force to another mass? The same question could be asked of attractive e-m forces as well. Don't these violate the conservation of ... 0answers 29 views ### Can you do an n-body simulation in terms of energy and momentum? An N-body simulation typically works directly in terms of the gravitational forces and accelerations. If you can solve the equations exactly, this is fine, but there are many instances where this is ... 2answers 105 views ### Optimal curve for a marble drop [closed] I'm doing a marble roller coaster project for my physics class at school. The first part of our roller coaster involves a marble falling downwards into a curve that will drop then go up 12 inches. ... 2answers 81 views ### Inelastic collisions During inelastic collision the kinetic energy does not remain constant. In many texts, I have seen it is because there is loss in energy from the system. If there is loss in energy, then how can mass ... 1answer 83 views ### What is the reason behind the equation$p = mv\$? [duplicate]

Now this is a very basic question, might look stupid too, but I am not able to understand it. I tried to imagine what momentum really is, and it is the impact of an object. I understood how momentum ...
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### Annihilation process and photons [closed]

Why should two photons produced by the annihilation process move in opposite direction? I know you would say for the conservation of momentum but why can't they move in the same direction, I want to ...