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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Particle anti-particle annihilation and photon production

This is just a conceptual question I guess. The annihilation of a particle with a finite mass and its anti-particle cannot lead to the emission of only one photon, and this is due to the conservation ...
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2answers
80 views

The acceleration of the centre of mass [closed]

Given two balls that are thrown straight up in the air at a speed of $40 \:\mathrm{m/s}$, $0.10 \:\mathrm{s}$ apart. One ball is twice the mass of the other. The heavier ball is thrown second. ...
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0answers
57 views

Why does gravity conserve linear momentum but not forces at a pivot point?

What forces count as an external force in the conservation of linear momentum? Consider an initially stationary rod that can rotate about one end-point. A ball comes in at a constant speed and ...
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3answers
67 views

Force of an object “skidding”?

Let's say I put the brakes on really suddenly in a car that was moving at 40mph. If I know how much force it takes to break the adhesion of the wheels, how do I find the force the car is exerting? ...
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0answers
29 views

The momentum representation of $x$ and $ [x,p]$ [duplicate]

To deduce the momentum representation of $[x,p]$, we can see one paradom $$<p|[x,p]|p>=iℏ$$ $$<p|[x,p]|p>=<p|xp|p>−<p|px|p>=p<p|x|p>−p<p|x|p>=0$$ Why? If we ...
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1answer
90 views

Conservation of energy or conservation of momentum- which one is applicable in this problem?

There was this question from Kleppner and Kolenkow's Classical Mechanics: PROBLEM: And I found its solution from this Link: It is quite an extended solution and I am not drawing you into the ...
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1answer
153 views

What is the initial velocity of an object, given a change in mass, and an exerted force [closed]

An 8.0-g bullet is shot into a 4.0-kg block, at rest on a frictionless horizontal surface (see the figure). The bullet remains lodged in the block. The block moves into an ideal massless spring and ...
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2answers
81 views

What happens to the kinetic energy of a system in a non-elastic collision? [duplicate]

I know that the kinetic energy of a system is NOT conserved in a non-elastic collision. But energy is supposed to be conserved, so where does all that energy go? Is it transformed into other forms ...
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1answer
98 views

Switch from the position representation to the momentum representation

If we use Fourier Transform, we can switch from the position representation to the momentum representation, like the following formula here comes the problem, if we use dirac notation we can see it ...
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1answer
31 views

Statics of a rigid solid [closed]

I need help with this problem... I need to calculate the angle so that this whole thing is in equilibrium. I don't have any problem with the conservation of the force. But when I get to the ...
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0answers
30 views

Why does the angular average of the divergence result in a factor 1/3?

In going from eq.(6.13) to (6.14) in http://www.nano.northwestern.edu/intranet/pubdocs/quasiclassical.pdf it is assumed that ...
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0answers
56 views

What is the effective electron mass in a plasma?

Is there a way to calculate the effective electron mass in plasmas? Effective electron mass is usually defined as varying from the vacuum electron mass in solid state physics. However, momentum ...
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1answer
126 views

Why is only angular momentum conserved for a planet and not linear momentum?

Suppose a planet is moving in an elliptical orbit with the Sun at one of its focii. I know that the forces in existence will be gravity which provides the necessary centripetal force. Now my book ...
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1answer
58 views

Understanding elastic collisions of objects with the same velocities

I am having trouble understanding the following statement from my book: The law of cosines tells us that if the sides of a triangle obey the Pythagorean formula, they must form a right triangle. ...
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1answer
66 views

Questions about the velocity of the center of gravity

I have a physics test coming up in a couple of days and I really need to have some things explained to me. It is about something that I think is called in english "the velocity of the center of ...
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0answers
41 views

Why is the angular average of the direction of the momentum squared a third instead of one?

In the article http://www.nano.northwestern.edu/intranet/pubdocs/quasiclassical.pdf (in going from eq. (6.13) to eq. (6.14)) it is stated that ...
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1answer
75 views

Why can we model a body to have a center of mass?

I accept that bodies have a center of mass. I also accept that if I were to suspend a body by only supporting it at its center of mass, then the body wouldn't rotate. In the scenario that we suspend a ...
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1answer
80 views

4-Momentum conservation for particle annihilation

Disclaimer: this is a homework question, so I am happy with just a hint or the expressions needed to proceed with my understanding. I am working on the momentum conservation of a ...
5
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1answer
221 views

How does one account for the momentum of an absorbed photon?

Suppose I have an atom in its ground state $|g⟩$, and it has an excited state $|e⟩$ sitting at an energy $E_a=\hbar\omega_0$ above it. To want to excite the atom, one generally uses a photon of ...
2
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1answer
138 views

Why does the rule that elastic collisions are at 90 degrees in 2 dimensions not apply?

When one object collides with another object of the same mass in a 2D plane, we know that we can derive that the angles that the objects leave the collision at add up to 90 degrees in a perfectly ...
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2answers
54 views

Invariance and conservation

Why in a collision between particles is the four-momentum conserved within a frame of reference but not invariant between frames of reference?
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1answer
150 views

Is momentum truly conserved? [closed]

Suppose I have a system of a person holding a rock on a frictionless sheet of ice. He throws the rock at an angle above the horizontal to propel himself off the ice. Is the momentum of that system ...
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1answer
47 views

Trouble Understanding the Concept of Invariance

I am reading the book by Brian Cox & Jeff Forshaw: "Why does E=MC^2". It is getting to the point of explaining what invariance is. According to Cox & Forshaw, invariance states that the laws ...
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1answer
44 views

Does energy flowing from one place to another in an object cause displacement of that object?

Let's consider two situations: a) A solid metal box (not hollow) where some heat is put on its left side. After some time, the heat will have moved to the right and also have spread out throughout ...
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3answers
80 views

Path followed by centre of mass of a ball when ball breaks up in two parts due to internal stresses

A ball was moving along a parabola and it breaks up into two parts. How can I say that the centre of mass will follow the same parabola ? I know that ( for simplicity I am assuming that the ball is ...
2
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0answers
68 views

Derivative of the action integral [closed]

I need to find the partial derivative of the action $S$ with respect to the generalized coordinate $q(t_f)$ and according to my textbook, it should equal the generalized momentum $p(t_f)$. I ...
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2answers
41 views

Why time of flight remains same for a projectile even though collision takes place?

Why time of flight remains same for a projectile even though collision takes place ? Suppose a ball is projected with some velocity V with some angle θ the the time of flight is T. Now if another ...
3
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4answers
267 views

Conservation of momentum in a baseball

Conservation of momentum: A thought experiment. A baseball is placed on top of a baseball holder, the kind used to train young batters. A batter hits the stationary ball perfectly horizontal, sending ...
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2answers
56 views

How can make myself sure that I can apply Conservation of Momentum in a system?

How do I know I can apply Conservation of Momentum in the System ? And What happens if there is a Impulse in the system?
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2answers
39 views

Stored Energy in My Badge Reel Finds Equilibrium With Centrifugal Force?

Probably a most horrible question title, please respond with suggestions for a better one. In the example of my badge reel, I grab hold of my badge itself and pull on the reel until the string is ...
2
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1answer
102 views

Area of phase space of Harmonic oscillator

We all know that the phase trajectory of an undamped linear harmonic oscillator is an ellipse. But when we calculate the area of the ellipse we find it does not depend of mass of the particle. Why is ...
2
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2answers
64 views

Why is it easier to drop on to a downslope?

On a bicycle, why is it easier to land from a drop or jump on a slope going downwards than landing on a flat surface or on an upslope? I've already heard answers such as "because that's how a bike ...
2
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0answers
51 views

Finding the momentum of a muon in $\pi\to\mu\nu_\mu$ [closed]

The $\pi$-meson's mass is 139.57 MeV, and it decays into a muon and a neutrino. The muon has a mass of 105.4 5MeV, and a proper lifetime of $2.197 \times 10^{-6}$. The neutrino can be treated as ...
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2answers
109 views

Problem regarding bullet penetration [closed]

Which property remains constant when we say that bullets are penetrating the wall? I mean is the resistive force always constant for a given wall (assumed stationary and immovable) irrespective of the ...
4
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4answers
143 views

Translational invariance implying diagonal representation in momentum space

I have just come across something in my reading of Peskin and Schroeder that claims that because a function, in this particular case a two-point correlation function, is translationally invariant, it ...
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1answer
100 views

Isolated system and conservation of momentum

I have a problem to understand isolated systems..If I have a ball and throw it upwards..then the momentum is not constant because the system is not isolated...and the reason is the external force from ...
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0answers
75 views

Four-momentum and Dirac equation in curved spacetime

Norm of four momentum in Minkowski spacetime is proportional to the square of rest mass as \begin{equation}|P|^2= P^\alpha \eta_{\alpha\beta}P^\beta= (E/c)^2 - p^2 = (mc)^2 \end{equation} While in ...
2
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0answers
80 views

Equating $\frac{dp}{dt}$ and $F$ at a hydraulic jump?

A journal I am reading about circular hydraulic jumps provides the equation and I quote: $H$= Depth after hydraulic jump, $h$= depth before hydraulic jump, $V$= velocity after hydraulic jump, $v$= ...
2
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3answers
85 views

Justification of $P_{\text{photon}}=E/c$ in derivation of $E=mc^2$

I recently was reading up on the derivation of $E=mc^2$. Now, I came across this derivation at this link. I noticed that several lines into the derivation they throw in the equation ...
3
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2answers
260 views

Estimate for energy dissipated by a damper/dashpot

I have a system with a mass $m$ attached to the end of a cable. The cable mass is assumed negligible. The cable is attached to the ground at the one end while the other, with the attached mass $m$, ...
1
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1answer
51 views

Is this treatment of the momentum operator in the Dirac formalism allowed?

I have a problem understanding a specific bit of Dirac notation. Take, as an example this derivation: I'm dubious about the step from line 3 to 4. When momentum operator acts on the momentum ...
1
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1answer
96 views

Do photons have no mass? [duplicate]

My Quantum Mechanics' teacher said today on the class that photons don't have mass. I was puzzled because I knew that photons have momentum. If a particle hasn't mass then its momentum sould be $0$ ...
1
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2answers
93 views

How force is transferred from one body to another

If there are 3 coins , namely 1 , 2 and 3 as in figure. When coin $1$ strike coin $2$ ,the coin $2$ passes the force to coin $3$ and the coin $3$ moves away. Case :1 How does this happen? What ...
1
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4answers
131 views

Why is force called the rate of change of momentum? [closed]

Why is force called the rate of change of momentum? If I push a wall I do exert a force but there is no movement; so is there force acting on the wall?
1
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1answer
101 views

Momentum and collisions [closed]

Two blocks of masses $m_1$ and $m_2$ connected by a spring of force constant $k$.The block $m_1$ is suddenly given a velocity $v_1$. Find the work done by the spring on both the blocks when it has ...
1
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1answer
135 views

Wavefunction interpretations in QM

From two-slit electron-interference experiment we can infer that there is a wave $\psi(x,t)$ that can be associated with electron. The amplitude at some point is the sum of amplitudes reaching that ...
0
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1answer
48 views

Physics Problem: Falling Rocket [closed]

A rocket, of mass $M = 40kg$, falls vertically with a speed of $60 m/s = v_0$ and explodes in two fragments of the same mass when it's at $H=2km$ from the ground. Immediately after the explosion one ...
0
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1answer
66 views

Momentum conservation in a small time interval

I recently solved a problem in which I used the "fact" that the momentum is conserved just before and just after a collision between two (or maybe more) objects but I am not sure whether this is true ...
0
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2answers
75 views

Conservation of momentum - finding the angle between broken parts

Let a stationary block explodes into 3 equal pieces .The speeds are also equal which is $v$. If I am asked to determine the angle between the broken pieces that are moving then how to deal with this ...