Questions tagged [conservation-laws]

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

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How does an accelerating mass on a ring interact with the ring?

In outer space, imagine a mass $m$ on a ring with mass $M$. The small mass is given a velocity $v$ after which it moves without friction on the ring. Initially, the velocity of the center of mass (COM)...
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Compression of airbags

I am currently creating a simulation of a rover landing on mars. Part of the landing would include the rover being dropped to surface of mars whilst surrounded by airbags. I understand that using ...
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Rocket's momentum

It is well known that to give a liftoff a rocket, we use the momentum principle: The momentum of the gases emitted by a rocket and the rocket's momentum is equal to zero. So why is it not enough to ...
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Is equation of continuity valid, while dealing with vertical pipes?

Let try an experiment. If water enter through an end $A$ with some velocity say $v_1$,and leaving end $B$ with speed $v_2$ in a UNIFORM cylindrical tube $AB$ (which is completely filled with water). ...
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Does the movement of a passenger change the velocity of an aeroplane?

I encountered a problem in my physics textbook today. An aeroplane of total mass of 50,000kg is travelling at a speed of 200m/s. If a passenger of mass 100kg then walks toward the front of the ...
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Block slides down frictionless wedge on frictionless surface. Find accelerations of the blocks

I am working on a physics problem where a small block of mass $m$ slides down a wedge-shaped block of mass $M$ with an angle $\alpha$ to the horizontal that rests on a frictionless horizontal surface. ...
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Is intrinsic spin conserved?

In general, we have total angular momentum, which is the sum of spin angular momentum and orbital angular momentum, and we know that in isolated systems the total angular momentum is conserved. Now, ...
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How is momentum conserved in the case of a ballistic pendulum collision?

To my knowledge, momentum is conserved when net external force equals zero. In the case of a block $m_1$ attached to a string hitting another block $m_2$, there must be a net force on $m_1$, which ...
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How can Hawking radiation maintain the balance of matter and antimatter?

If hawking radiation simply reduces the mass and emits electromagnetic radiation, how can the balance of matter / antimatter be maintained? Couldn't you use some of that $E = mc^2$ energy you would ...
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Finding symmetry from conserved quantity (reversed Noether's theorem) [duplicate]

Is Noether's theorem reversible? For example, given some conserved quantity, can you find the underlying symmetry which leads to the conserved quantity?
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First-order EM Feynman diagram?

Is there any 1st order electromagnetic Feynman diagram? I.e. a process whose probability is just $\propto \alpha_{EM}$? If not, is there any physical reason why? We always need at least two particles ...
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How to use the concept of variable mass system?

The general equation of variable mass motion is as follows: It's derivation is given below: Refer wikipedia page for more details and the link of it is given below: https://en.wikipedia.org/wiki/...
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Why can't we use conservation of angular momentum to solve this question since this whole system is isolated?

A solid rubber wheel of radius $R$ and mass $M$ rotates with angular velocity $\omega_0$ about a frictionless pivot. A second rubber wheel of radius $r$ and mass $m$ also mounted on a frictionless ...
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If everyone on Earth jumped at the same time, is it possible to steer the Earth?

Newton's third law states that in every action, there is a equal and opposite reaction. When we jump, we exert a force on the ground and the resultant force upwards propels us up into the air. However,...
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How to define conserved charges in Euclidean field theory?

In a field theory with signature (1,d), conserved charges are obtained by integrating the time component of a conserved current over a spatial region. What are the corresponding equations and ...
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How is energy conserved in blueshifting?

I am having trouble understanding the conservation of energy in the scenario below. A photon of $\lambda_1$ is emitted by a source and then absorbed by object 1 in space. Object 1 can be considered ...
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Why does a spinning top get back upright when knocked?

I have observed that, a spinning top, when toppled by knocking it with finger, becomes almost upright again (with negligible precessional motion), instead of undergoing precession while being heavily ...
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3answers
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Friction during rolling

I have four doubts regarding friction during rolling. Does slipping mean zero angular velocity, or is it just the $v$ velocity not being equal to $\omega r$? If a wheel is initially given the ...
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1answer
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Why is angular momentum conserved if we keep a ball on rough surface with only translational velocity after it attains pure rolling motion

Assume a ball to be kept on a rough surface with intial translational velocity V and no rotation. After some time it aquires pure rolling. It is acted upon by an external torque due to friction still ...
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Conservation of angular momentum and central forces

We review the statement and derivation of the law of conservation of angular momentum of a system of particles. $\textit{Theorem:}$ Consider the system of particles $$S := \{ P_i | P_i \; \text{is a ...
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1answer
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Momentum conservation in FRW spacetime

The spatially flat FRW metric in Cartesian co-ordinates is given by: $$ds^2 = -dt^2 + a^2(t)(dx^2 + dy^2 + dz^2)$$ As I understand it, since the metric does not depend on the spatial co-moving co-...
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Conservation of Charge vs. Conservation of Energy

Is conservation of charge ever violated like conservation of energy is violated during cosmological expansion? I am trying to understand this with respect to Noether's Theorem.
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Free-body diagram for a collision

Edit - Consider an homogeneous bar of length $L$ and mass $M$. This bar can rotate on a horizontal plane with no friction around a point $A$. The distance between $A$ and the center $O$ of the bar is $...
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1answer
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How to understand the Fresnel relation $1+R=T$? [closed]

From the perspective of energy conservation, we are familiar with the relation $T+R=1$ (Set the incident wave amplitude as 1, $T$ and $R$ are Fresnel transmission and reflection coefficient, supposing ...
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1answer
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Amplitude of a wave, and boundary effects

I am curious as to what causes a discrepancy in the amplitude between the incident and the reflected/transmitted pulse. (* For instance, in the first image, the incident pulse seems to be of greater ...
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1D Elastic Collision with restitution coefficient

I have an exercise about a 1D collision with a certain restitution coefficient, that is: $$e = \frac{|u_1-u_2|}{|v_1-v_2|}$$ One must calculate the velocities of the 2 colliding masses after the ...
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Energy conservation in systems with discrete time

Numerical simulations of classical particle dynamics usually break energy conservation due to discretization of time. Is there any explicit numerical scheme that does not break energy conservation (...
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Interpreting the conserved charge in scalar QED

In scalar QED, applying Noether's theorem for internal global symmetries results in a Noether current that is dependent on the gauge because of the presence of the covariant derivative. $$j_\mu=-i(\...
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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 of ...
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1answer
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How impulse momentum theorem is put to use?

Guys the below link is a question already asked in this website and I have referred the answers given by the users and some of them were good ,but I have few doubts and I wanted to ask it in a comment ...
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1answer
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Doubt On Maxwell's Stress Tensor

I was reading Introduction to Electrodynamics By D.J. Griffiths Chapter 8 Conservation Laws , Maxwell's Stress Tensor.The starting lines are the following Let's calculate the total electromagnetic ...
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1answer
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Conservation of Energy with Chemical and Kinetic Energy of Moving Body

A rocket is moving relative to the earth such that it has $E$ joules of kinetic energy. The rocket contains fuel with $E$ joules of chemical energy. The mass of the fuel is negligible in comparison to ...
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Conservation of momentum: One moving car hitting another stationary one. Does the target always move forward?

Consider an isolated system with two cars of any mass, a ground with friction, and Earth. Both cars are free to move on the ground. One car (Car #1) is moving towards the other stationary car (Car #2)...
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If electrons can be created and destroyed, then why can't charges be created or destroyed? [duplicate]

I read on Wikipedia that electrons can be created through beta decay of radioactive isotopes and in high-energy collisions, for instance when cosmic rays enter the atmosphere. Also, that they can be ...
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What is the significance of the term 'isolated system' in law of conservation of charge?

I did not understand why charge can only be conserved in an isolated system. Kindly elucidate.
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Why is angular momentum $mvr$ and not $mvr^2$ or $mvr^3$?

We know angular momentum $L = mvr$, where $v$ is the velocity in the direction perpendicular to the distance from the source to the object whose $L$ we are trying to measure. My question is why $L$ ...
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1answer
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Forces in an elastic 1D collision [closed]

Let's say I have block A (5kg, a = 10 $\frac{m}{s^2}$) and block B (20kg, a = 20 $\frac{m}{s^2}$). When they collide together elastically, the forces exerted on each other should be the same by Newton'...
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Conservation of Charge

We know that universe is all about charge conservation mass conservation and energy conservation. Charge on a system can be measured by comparing it with charge on a standard body, positive and ...
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How can I show that in an elastic collision i can set $| \vec{v_2} - \vec{v_1} | = | \vec{v_2}' - \vec{v_1}' | $?

I am getting stuck in a really easy problem in Statistical Mechanics that involves elastic collisions, it is really very shameful that I am getting stuck at such a simple thing, but from: $$\|\vec{v_1}...
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Kinematics of Decay of Ultra-Relativistic Particle

An ultra-relativistic particle with $pc>>Mc^2$ is supposed to decay into two massive particles of mass $m$. I've been told that this is impossible and have tried the following to reach that ...
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Conservation laws for the collision of two classical particles

In Kleppner and Kolenkow's An Introduction to Mechanics (3rd ed.), the authors state (p. 227) that for the collision of two classical particles with no external forces or torques, with 6 unknowns (the ...
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Do conserved quantities tell us more general pattern than what we are taught?

Last year I had a question in m mind that as $mv$ (momentum) and $mv^2/2$ (Kinetic energy) both are conserved then n a closed system and they are dependent on same quantities and look very similar so ...
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How to interpret charge continuity equation for conductors that obey Ohm's law?

For conductors, we propose that the free current density is proportional to the applied Electric field and the constant of proportionality is defined as conductivity. \begin{equation} \textbf{J}_\...
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Noether's Theorem: There are conserved quantities corresponding to symmetries of position, orientation, and time, but why not velocity?

Noether's Theorem seems to be one of the most fundamental and beautiful results in all of physics. As I understand it, the fact that the laws of physics are the same independent of position, ...
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1answer
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How do higher-order optical chiralities look like?

The optical chirality of the electromagnetic field is a conserved quantity, analogous to the energy density, linear momentum density, and angular momentum density, which describes how chiral the EM ...
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What symmetry is associated with conservation of Lipkin's zilch?

The 'zilch' of an electromagnetic field is the tensor $$ Z^{\mu}_{\ \ \ \nu\rho}=^*\!\!F^{\mu\lambda}F_{\lambda\nu,\rho}-F^{\mu\lambda}\,{}^*\!F_{\lambda\nu,\rho} \tag1 $$ given in terms of the ...
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Problem with the continuity equation for an electron gas

Consider the continuity equation for an electron gas: $$\tag{1} \nabla \cdot\left[n(\boldsymbol{r}, t) \frac{\partial}{\partial t} \tilde{\boldsymbol{r}}(\boldsymbol{r}, t)\right]=-\frac{\partial}{\...
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Photon hits an electron perpendicular to its velocity, Relativity and Work?

In the phenomenon of the Compton scattering a photon can hit a free electron under any angle. The photon can be regarded as a 'complex' of two photons one along the velocity v of the electron and ...
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How can a planet gravitationally capture objects?

I would expect that any asteroid or other object originating far away but passing near a planet would pick up speed and energy as it approaches, but unless it comes into contact with the atmosphere (...
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Energy imbalance during conservation of momentum associated with the collision of a molecule with the wall of a container

I've read a couple of answers related to this topic, but none of them answer my specific question. My question is that, when a molecule collides with the wall of a container, having $X$ component of ...

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