Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

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.

1
vote
0answers
12 views

If you set crates of different masses to the same initial velocity, which one will stop first and which one will have the greater displacement? [on hold]

If you set two crates of different masses to the same initial velocity, which one will take the longest amount of time to come to a stop? Which one will travel the longest distance? Which one will ...
1
vote
3answers
90 views

Why equation $F = m a$ is not valid in relativistic physics but $F = dp / dt$ is valid?

I read that the equation $$F = m a$$ is not valid in relativistic physics. Instead $$F = \frac{d }{dt} \frac{mv}{ \sqrt{1- \frac{v^2}{c^2} } }$$ is valid in relativistic physics but why is that?
3
votes
0answers
20 views

Angle between velocities: Elastic collision question in 3D

I have been given a scenario of a proton (particle 1) travelling with u1 = 3î+ 4j–6k and a Helium-3 nucleus (particle 2) travelling with u2= 3î+ 4j–2k. The two particles collide and the proton (...
0
votes
1answer
29 views

Find impulse provided by the stopping object [on hold]

I have tried to understand this problem, but I'm still struggling with it. a 27g bullet hits a piece of wood and goes in 8.6 cm. if the bullet stopped in 0.44ms. what impulse did the wood provide?
2
votes
5answers
135 views

Conservation of mass in relativistic collisions?

It's stated in my textbook that relativistic mass is conserved in collisions, even in inelastic ones. So if you have a particle with rest mass $m$ moving with speed $u$ (considerable fraction of the ...
0
votes
1answer
51 views

Which rifle will hurt the shoulder more and why?

I have this: Two identical bullets are fired one by a light rifle and another by a heavy rifle with the same force. Which rifle will hurt the shoulder more and why? and my thoughts about the answer ...
2
votes
2answers
60 views

Why can't we expand the definition of the system until momentum is conserved?

So, here a ball initially moving with a velocity v and collides inelastically with a pivoted rod. I've learned that this is a classic example for the conservation of angular momentum. My question is ...
-1
votes
1answer
58 views

What is the difference between space-like four-momentum and time-like four-momentum?

I was reading a wiki page on Tachyon, came across these terms? What i need is bit of a mathematical description to understand these terms?
-1
votes
1answer
44 views

Violation of law of conservation of linear momentum

A bike is moving with some velocity. Consider that there is no frictional force between the wheels of bike and ground. Now you apply brakes and the bike stops after some time(bike will stop due to ...
1
vote
0answers
36 views

Angular momentum of a system about the center of mass

Let $\boldsymbol{R}$ be the center of mass of a system of particles. Then the angular momentum of the system is $$\begin{align} \boldsymbol{L} &= \sum \boldsymbol{r}_i \times \boldsymbol{p}_i\\\\ &...
-1
votes
0answers
34 views

Velocity Question

I've attempted this question for my homework. Can you tell me if I'm correct as I'm not too sure. Any help would be appreciated if I'm wrong. Here's the question: https://imgur.com/a/qD3symx Here's ...
1
vote
0answers
26 views

Neutrino's relativistic four-momentum

In an exercise I have to solve there is an apparently contradiction that I can't figure out. In a pion decay, where a muon and a neutrino are produced, conservation of quadric-momentum states: \begin{...
1
vote
2answers
53 views

Ambiguity in d'Alembert's principle

It seems to me that many different momenta $\dot{\bf p}_j $ can satisfy d'Alembert's principle: $$\tag{1} \sum_{j=1}^N ( {\bf F}_j^{(a)} - \dot{\bf p}_j ) \cdot \delta {\bf r}_j~=~0 $$ in a ...
-1
votes
0answers
27 views

When a rocket applies a force to a mass (a gas) in one direction does the rocket experiences a force in the opposed direction? [on hold]

When I fire a gun the exploding charge applies a force to the bullet. I feel a recoil force on my shoulder in the opposed direction. This conserves momentum? The total momentum was zero before the gun ...
1
vote
4answers
49 views

Formula for potential energy? Conservation of energy?

How would we know what formula to use for potential energy? In my class, $mgh$ was used, but when dealing with a spring, it's ${1\over2}kx^2$. Is that because that's the elastic potential energy ...
7
votes
2answers
479 views

Car and truck collision

When a car and a truck crash, the car seems to get damaged a lot more. Why does the car get more damaged then the truck in the collision?
0
votes
1answer
47 views

Proof of overall momentum conservation

In $\phi^4$ momentum conservation at each vertex comes from integration in the vertex. How can from momentum conservation at each vertex proof overall momentum conservation that is if $k_1,k_2 ,k_3,...
12
votes
3answers
763 views

Elementary argument for conservation laws from symmetries *without* using the Lagrangian formalism

It is well known from Noether's Theorem how from continuous symmetries in the Lagrangian one gets a conserved charge which corresponds to linear momentum, angular momentum for translational and ...
3
votes
2answers
62 views

Can a particle radiate in vacuum without external forces applied?

I was reading about possible and forbidden processes involving photons. First of all, the annihilation process of a pair electron and positron cannot result into a single photon because it would be ...
4
votes
5answers
3k views

Why isn't energy conserved in this collision problem? [closed]

I am stuck in a question where energy conservation is failing and momentum conservation is correct. I think I might be doing something wrong, that's why I'm asking this question. The problem is as ...
1
vote
0answers
25 views

Collision of light with light [duplicate]

Photons have momentum. So when a light passes through another light what happens? Do they collide and conserve momentum? If they collide they should deviate (move away from the straight line at a ...
1
vote
1answer
28 views

Wave function - expected momentum is infinite

I have a wave function, say $$ \begin{align} \Psi(x) = xe^{ikx} \end{align} $$ And I want to find the expectation value for momentum, $<p>$ It's not working out to an integral I can evaluate ...
2
votes
1answer
31 views

Confused why momentum is conserved in 2 falling bodies problem [duplicate]

I was doing a physics problem out of my textbook, which could paraphrased as follows: There’s a frame suspended at rest from a coiled spring. A mass of flexbile putty is dropped onto the frame, ...
-2
votes
1answer
35 views

Momentum Operator for Schrödinger's equation notation

I'm studying Chemistry but we are studying Quantum Physics as a separate module and I had a question regarding the momentum operator. In the lecture, we were given the following information: The ...
6
votes
1answer
169 views

Is there a function which is square integrable and doesn't tend to zero at infinity but it belongs in the domain of the momentum operator?

Is there a function which is square integrable and doesn't tend to zero at infinity but it belongs in the domain of the momentum operator? There are some counterexample for functions that are square-...
1
vote
2answers
36 views

How is force related to launching objects with a lever?

Suppose we have a class 1 lever, lifting a load of constant mass. All distances remain the same and the only thing that changes is the magnitude of the force applied (effort). Intuitively we know that ...
0
votes
1answer
35 views

If two identical objects (A and B) are dropped from the same height, and B has protective packaging, why is B less likely to be damaged?

I know this question has to do with impulse and collision, and if it were in numbers, I could give an answer to it, but how can I explain this in words? Both object A and B are identical, however, ...
1
vote
0answers
151 views

Does “Quantized Inertia Theory” violate conservation of momentum? [closed]

This recent news article reports that DARPA is doing work with "quantized inertia", despite their claim that it's not widely accepted by physicists: The Defense Advanced Research Projects Agency (...
-1
votes
0answers
27 views

Spring assisted jump [duplicate]

If you wanted to jump as high as you can now, but without making any effort, you could use a compressed spring to provide the push you need to jump. If you had a spring with a low spring constant, how ...
1
vote
2answers
56 views

Does the mean value change?

We know that $x_0$ and $p_0$ are the mean values for the position and momentum of a particle in the normalized state characterized by the function $\psi (x)$, ( that is, $x_0=\langle x \rangle_\psi$ ...
1
vote
2answers
55 views

Conservation of momentum on moving charge

The forces of attraction or repulsion between two stationary charges are the action and reaction forces between them. So when we move one of the charges then the other charge also gets affected. But ...
1
vote
2answers
58 views

Defining generalized momentum in terms of kinetic energy versus a Lagrangian

Reputable authors (e.g., Bergmann, Wells, Susskind) define generalized momentum using the Lagrangian $L$ as $$p_{i}\equiv\frac{\partial L}{\partial\dot{q}^{i}}.\tag{1}$$ Joos and Freeman define ...
1
vote
4answers
86 views

Momentum of an ultrarelativistic electron

I am aware of the relativistic equation: $E^2 = (pc)^2 + (mc^2)^2$ And if we are dealing with a massless particle then $E = pc$ However I am doing some work in Astrophysics and have been told that ...
0
votes
0answers
15 views

How to equate the string tension of a racquet to coefficient of restitution?

I'm a tennis fan and wanted to see the relationship between the string tensions of the racquet and the velocity of the ball after it bounces off the racquet. I used the coefficient of restitution ...
0
votes
3answers
87 views

How to derive derive De-Broglie's wavelength equation?

I was only yesterday learning about the De-Broglie equation $$\lambda = h/p, $$ I thought I understood it until I came across a question similar to this Now I always knew that photons don't have any ...
2
votes
2answers
89 views

Are these collisions equivalent?

Similar to the question if two cars with a velocity of 50 mph each colliding is the same as one car colliding with wall at 100 mph, I was wondering if the same amount of energy is produced when ...
1
vote
0answers
27 views

Gauge freedom in Lagrangian corresponds to canonical transformation of Hamiltonian

I want to show that the gauge transformation $$L(q,\dot{q},t)\mapsto L^\prime(q,\dot{q},t):=L(q,\dot{q},t)+\frac{d}{dt}f(q, t)$$ corresponds to a canonical transformation of the Hamiltonian $H(p, q, ...
5
votes
5answers
145 views

Newton's Third Law and conservation of momentum

I was looking through some reference material, and I came across this: In the situations in which we cannot follow details, we need to know some general properties, that is, general theorems or ...
8
votes
3answers
339 views

Does light lose energy when it reflects from a surface?

When light reflects from a surface, at least the direction of its momentum changes. Since the total momentum must be conserved, there has to be something going on within the atoms of the surface. So ...
0
votes
1answer
36 views

Is the coefficient of restitution negative?

I recently came across a formula for the coefficient of restitution: Why is there a negative sign?
0
votes
0answers
31 views

Get relation from definition of stress-energy tensor and the conservation of energy

Starting from the following definition of stress-energy tensor for a perfect fluid in special relativity : $${\displaystyle T^{\mu \nu }=\left(\rho+{\frac {p}{c^{2}}}\right)\,v^{\mu }v^{\nu }-p\,\eta ...
1
vote
0answers
53 views

Does the angular momentum of the cornering/turning car change the travel direction of the wheels of the car?

I think this is purely a physics question and does not require any deep knowledge of vehicle dynamics. The cornering car has angular momentum with a rotation around a vertical axis but when you ...
2
votes
1answer
62 views

Weinberg's Lectures on Quantum Mechanics - Definition of Momentum Operator

In Weinberg's "Lecture on Quantum Mechanics" (2nd edition, page 79) in equation 3.5.11, about the momentum operator acting on states definite position, the minus sign is missing. Is this just a typo ...
1
vote
0answers
33 views

Newtonian Mechanics - Regarding Conservation of Momentum in Collisions

In the question, it is given that all three balls are identical and are connected by a massless, ideal string (on a horizontal plane), and initially, the ball at the centre is given a velocity $v$. If ...
0
votes
0answers
25 views

What's the most efficient way to walk and swim?

I hope this is the correct forum for this question. I have two related questions that deal with the same issue. My primary question is regarding swimming. I've found that extending my stroke (...
1
vote
2answers
118 views

Definition of generalized 4-momentum

In nonrelativistic mechanics, given a lagrangian $L$, we define the action as $$S[q]=\int L(q(t),\dot q(t))dt\tag{1}$$ and we can prove (see, for example, this answer for eq. (2)) $$\begin{align} \...
0
votes
1answer
62 views

Expected momentum of ground state hydrogen $<p>$

I am trying to calculate the expected momentum of an electron in the ground state of hydrogen atom. This is the wave function. So far I have done this:$$\iiint_V \Psi^* (-i\hbar) \frac {d\Psi} {dr} ...
4
votes
4answers
828 views

How is momentum conserved in this example? [duplicate]

Suppose a sticky substance is thrown at wall. The initial momentum of the wall and substance system is only due to velocity of the substance but the final momentum is 0. Why is momentum not conserved?
0
votes
0answers
44 views

Would the formation of the Mediterranean sea be enough to shift the Earth on its axis?

According to this article, the Mediterranean sea was formed when the Atlantic broke through (what is now) the Straits of Gibraltar, and flooded the basin. Assuming that is correct, and also that the ...
0
votes
1answer
28 views

Is angular momentum conserved in case of a vehicle turning?

When a vehicle takes a turn is the sharpness viz radius of turn related to the velocity by conservation of angular momentum ? If it is so then while going on a straight a line it has infinite radius ...