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Questions tagged [conservation-laws]

The statement that a property of a system does not change if the system is isolated.

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Moving a (specific)-spacecraft without ejecting mass

The idea is to move this spacecraft without ejecting any mass. The craft has two paddles which we can make spin. So we rev these two paddles up, spinning them in opposite directions and when their ...
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2answers
69 views

How propulsion in the “HELPING HAND” NetFlix episode works? *SPOILER*

I watched the "HELPING HAND" episode in NexFlix, and the protagonist is drift in space, every second she is more far from her ship, when her tank is out of oxygen, she throw her glove to make a ...
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How does the momentum conservation law hold for gravitational interactions in special relativity with Newton's third law being invalid?

Griffiths' argument that Newton's third law is invalid in special relativity The selected answer to the post above talked about momentum conservation in electrodynamics – Newton's third law ...
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Is the Lorentz transform of a bound current - a bound current?

The bound (magnetization) current is $\mathbf{J}_\mathrm{M} = \nabla \times \mathbf{M}$. The charge density of a moving magnet that is electrically neutral in its own rest frame satisfies the ...
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Can this sum be done without integrating?

As by using conservation of mechanical energy, we can just be concerned about the final and initial position and the velocity could be found out. Am I not right? But I am not getting the correct ...
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1answer
33 views

Confusion regarding conservation of momentum and conservation of energy [on hold]

When solving the problem under in the two different ways listed, i end up getting different answers. An electron is moving with $v_1=0.87c$ perpendicular to a positron moving with $v_2=0.98c$ in an ...
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1answer
45 views

Question about the angular speed of a rod as it falls to the ground [on hold]

I was given a problem yesterday in which a bird flies into a rod and then is stunned. It gives the rod some initial angular speed (found through conservation of momentum). However, my TA then claims ...
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4answers
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How can momentum but not energy be conserved in a collision, if objects keep moving in the same direction?

For example, there are 2 objects with the same mass, moving in the same direction, one with speed 5 m/s and the other 1 m/s. They collide and keep moving in the same direction, now one with speed 4 m/...
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Are there non-holonomic constraints that cannot be removed, other than conservation of energy?

I'm learning about Lagrangian mechanics at the moment. The idea that $ L(\mathbf{q}(t),\mathbf{\dot{q}}(t), t) = 0$ fascinates me. It strikes me as this could be viewed as a 1st order non-holonomic ...
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1answer
40 views

Conserved quantities in ballistic pendulum

A bullet of mass $m$ hits a sandbag of mass $M$ that is suspended from the ceiling by a massless rope. At the moment the bullet hits it has a horizontal velocity $v$, and it gets stuck in the sandbag. ...
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1answer
37 views

Centre of mass and conservation of momentum [closed]

A bullet of mass $50$ g moving with velocity $600$ $m$ $s^{-1}$ hits a block of mass $1$ kg placed on a rough ground and comes out of the block with a velocity of $400$ $m$ $s^{-1}$. The coefficient ...
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3answers
50 views

Is vector sum of momentum constant if there is friction?

In a head-on collision of 2 objects with the same mass and speed in opposite directions, the vector sum of momenta before and after the collision stays the same and is equal to zero. I've read that if ...
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6answers
2k views

Conservation of Mass and Energy [duplicate]

I was thinking about some physics (relativity in particular), when it suddenly occurred to me that all my life I had been balancing chemical equations assuming conservation of mass, but I was ...
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0answers
25 views

Rotational dynamics (conservation of angular momentum with friction) [closed]

This question is solved by conservation of angular momentum even though there is friction
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3answers
60 views

How does conservation of momentum work here? [closed]

Imagine 2 oppositely charged particles attracted to each other, say an electron and a nucleus, assume their force of attraction is very large compared with any external applied force we're about to ...
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2answers
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Do black holes violate the conservation of mass?

This question has confused me for quite some time now. I have searched it up online, and the basic answer is: 'Mass is a form of energy. When black holes die they release the amount of energy that ...
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3answers
91 views

How did the bike driver reduce his momentum?

Consider the following GIF The bike driver gained relatively large momentum from the ride. My question is, how the flip he made helped him reducing his momentum and landing safely?
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1answer
85 views

Lorentz force in superposition of two magnetic fields

When an electron with charge $q$ travels with velocity $v$ perpendicular to a magnetic field generated between two permanent magnets with field strength $B$ and no electric field, it experiences a ...
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0answers
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Can there be conservation laws without a corresponding symmetry? [duplicate]

Noether's theorem implies that if there is a continuous symmetry in the Lagrangian of a system, this necessarily implies a corresponding conservation law. But does the theorem also imply the reverse? ...
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1answer
23 views

Boundary terms and Symmetries

Consider Maxwell-Chern-Simons theory in 2+1 dimension, with Lagrangian $$L = -(1/4)F_{\mu v}F^{\mu v} + (m^2/4) \epsilon_{\mu v \rho}A^\mu F^{v \rho},$$ when I make a gauge transformation $A_\mu \to ...
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1answer
44 views

Conservation of Angular Momentum — Earth-Moon System — Earth spin backwards?

I am developing an exhibit for a museum. We want to show how the Earth's spin rate changes as the Moon drifts farther and farther out from the Earth. The visitor has a slider they can move to set ...
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2answers
65 views

Is the interaction $\gamma p \to \pi^+n$ allowed?

I'm doing an undergraduate lecture course in particle physics and I'm still getting to grips with the basics of interactions. One of the example interactions I've been given in an exercise is $\gamma ...
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1answer
58 views

Conservation of quasimomentum and added-mass: help needed to understand a derivation in G. Falkovich's fluid mechanics book

Context Starting at page 30 of Falkovich's Fluid Mechanics, there is an interesting discussion about the relationship between the conservation of quasimomentum in an isentropic ideal fluid and the ...
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1answer
41 views

Elastic collision between a circle and a point [duplicate]

In a 2D environment, I have a circle with velocity v, a stationary point (infinite mass), and I am trying to calculate the velocity of the circle after a perfectly elastic collision with the point. ...
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1answer
26 views

Expression of mass flow rate for steady flow

According to Wikipedia the mass flow rate is given by: $${\dot {m}}=\iint _{CS}\rho \mathbf {v} \cdot {\rm {d}}\mathbf {A}$$ And using the Reynolds Transport Theorem (if there is no sources): $$\...
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1answer
32 views

Solid state physics, meaning of momentum notated G in photon-phonon collision?

I have a problem in momentum conservation in the scattering of photon with phonon.In the process the momentum conservation is written as k'=k+G+q.where k'=wave vector of scattered photon,k=wave vector ...
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1answer
31 views

Calculating velocities of 2 squares after collision

If I have 2 squares each with random velocities, and both with the same mass, would it be possible to calculate their velocities after a collision, and how would I do so?
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1answer
56 views

Which formula for elastic collision?

When researching a bit about elastic collisions, I've been confused about how many different ways there are to do this. I've seen multiple methods, some of which are extremely complex (using sine and ...
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0answers
22 views

Showing conservation of momentum for arbitrary pendulum trajectories

Consider an isolated system of a pendulum driven by a motor, initially at rest. Conservation of momentum and angular momentum ordains that the center of mass and orientation cannot change in the ...
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0answers
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Can building nuclear reactor at some height fail the law that energy can't be created or destroyed

What if we built a nuclear reactor at some height we will have to provide some potential energy to the reactants but after the reaction the mass of products will be less than the mass of reactants and ...
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0answers
29 views

Lowest energy for outcome particles in two particle collision

I have an equation for two particle collisions: $$k_{A,x}+k_{B,x} = p_{1,x}+p_{2,x}$$ $$k_{A,y}+k_{B,y} = p_{1,y}+p_{2,y}$$ $$k_{A,z}+k_{B,z} = p_{1,z}+p_{2,z}$$ $$\sqrt{m_{1}^{2}+\mathbf{k}_{A}^{2}}+...
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3answers
67 views

How do rocket engines work in EMPTY space? [duplicate]

So I just read this and it didn't make any sense It is not necessary for the rocket exhaust to push against anything EXCEPT THE SHIP ITSELF. You see, when the combustion of fuel takes place ...
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2answers
35 views

Conserved and invariant quantities in a nuclear reaction

For a reaction $$a+X\rightarrow Y+b$$ in the book, Introductory Nuclear Physics by Krane, he has written that the total energy and the total momentum both in RHS and LHS remains conserved in Lab frame,...
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2answers
97 views

Continuity equation, differential form vs integral form in 1D flows

The continuity equation for fluid flow assuming steady-state and constant fluid density, that is: $\rho = \text{constant}$ and $\partial_t=0$ can be written in the two following forms (differential ...
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3answers
860 views

Why do objects rebound after hitting the ground? [duplicate]

When an object, say a shoe, falls from a height (under the influence of gravity), it rebounds after hitting the ground. For an object to move upwards, it requires a force to overcome its weight. When ...
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2answers
308 views

If an object in uniform motion with zero net force, can its mass change?

According to newton laws if something is in uniform motion and equivalent force is equal to zero then its mass cannot change. Our book says an object stays in state of rest or motion unless compelled ...
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4answers
111 views

Can a proton and an electron annihilate in a gravitational field?

According to this Physics.SE comment, it is gravitationally allowed, though very unlikely, for a proton and an electron to annihilate yielding two photons. Is that correct? If so, why? (In ...
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2answers
113 views

Why are symmetries in phase space generated by functions that leave the Hamiltonian invariant?

Hamilton's equation reads $$ \frac{d}{dt} F = \{ F,H\} \, .$$ In words this means that $H$ acts on $T$ via the natural phase space product (the Poisson bracket) and the result is the correct time ...
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conservation of momentum, a block on an inclined plane free to move

A block with mass $m_2$ is placed on a frictionless inclined plane with angle $\theta$ and mass $m_1$ which can freely slide on the floor. My question is relating the the velocities of the two mass. ...
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What's the most general form of Maxwell's stress tensor for EM fields in matter?

In Griffith's Introduction to Electrodynamics we find a derivation of the relation $$\mathbf f + \frac{\partial \mathbf g}{\partial t} = \nabla \cdot \overleftrightarrow{\mathbf T} ,$$ where $\mathbf ...
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1answer
52 views

Predicting elastic collisions [closed]

I want to predict how a collision will go with physics. Say I have a perfectly elastic 1-dimensional system with a frictionless ground and two objects of masses say $m_1$ and $m_2$ moving with ...
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1answer
33 views

Calculating Noether Current for Electromagnetic Current interacting with a Dirac Fermion

I'm trying to confirm that the conserved current of the Lagrangian $$ {L} = -\frac{1}{4}F_{\mu\nu}F^{\mu\nu} -j^{\mu}A_{\mu}+\bar\psi(i\gamma^{\mu}\partial_{\mu}-m)\psi $$ associated with the ...
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1answer
28 views

How do we define the quantity $Q$, in the conservation of energy? And what does it rely on?

Noether's theorem to me explains how a certain defined quantity (Q) is conserved (locally) in time due to the time translation symmetry, and to be more specific; if we had a ball that is placed in a ...
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1answer
52 views

Where does my mass go when i lose weight? [duplicate]

Presuming conservation of mass to be true, mass has to go somewhere if i lose weight. Doing workout i burn my fat to do some work but then where does that fat go?
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3answers
67 views

Is Poynting vector conserved or Intensity conserved for reflection and transmission of electromagnetic wave?

When an electromagnetic wave meets a boundary, does energy conservation mean Poynting vector of reflected + Poynting vector of the transmitted wave is equal to Poynting vector of incident wave or just ...
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1answer
49 views

Angular Momentum, and its Conservation

I am confused as to how to reconcile the conservation laws of linear and angular momentum. Are they independent (and how can this be, if $L = r$ x $p$?) Does one supersede the other? In short: how are ...
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1answer
29 views

How do I find the direction of impulse vector in a collision?

I know that impulse is the change in linear momentum. Is the direction of impulse always along the change in momentum vector? Please explain.
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1answer
64 views

Does the mass affect the speed if friction is 0

A block $m_1$ strikes another stationary block $m_3$ inelastically . Another block $m_2$ is kept on $m_3$.Neglect the friction Between all contacting surfaces. Since $m_2$ is kept on $m_3$ and all the ...
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1answer
103 views

Why is there equal protons and electrons in the universe?

Matter and anti-matter are asymmetric. Why are protons and electrons symmetric? Would we still have galaxies and stars if there were a strong overall charge?
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2answers
118 views

What goes wrong, theoretically, when we reverse time?

(Please bear with me if this is a stupid question; I'm not a physicist, just a curious student.) I know that Noether's Theorem links symmetries to conserved quantities: the fact that the laws of ...