Questions tagged [conservation-laws]

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

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Baryon number conservation in LHC

The LHC uses pp-beams in a colliding experiment. This means, their baryon numbers add but afaik the products do not carry these baryon numbers?
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Energy threshold for photon

I just read https://en.wikipedia.org/wiki/Annihilation#Examples and there is the popular $ e^+e^- \to \gamma \gamma $ reaction described. Is it always possible to produce a $ \gamma $? A photon does ...
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Why is $J/\psi$ not being able to decay into $\pi^+\pi^-\pi^0$ (Ozi-rule)?

There are already (very similar) questions like What is the precise statement of the OZI Rule? and Why does charmonium (and phi mesons) not decay via quark and antiquark annihilation? but, to be ...
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Why are certain quantities so fundamental to physics? [on hold]

Apart from having a qualitative description of quantities such as momentum, work and energy, why are these quantities considered so fundamental? What is the reason to define them in the first place? ...
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Angular Momentum — AP Physics C

Do all of the planets in our solar system have the same angular momentum? If so, why? I'm not sure if the following is correct: If we could drag a planet radially outward from the sun, then no ...
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How much energy, momentum, and/or angular momentum are lost to radiation in Rutherford scattering?

Inspired by this question: Consider two charged particles, of masses $m_1$ & $m_2$ and charges $q_1$ and $q_2$. They approach each other from a great distance, interact via their electromagnetic ...
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Will momentum be conserved in case of electrostatic force?

If a charged particle approached another particle of same charge held by some external force, then by coulomb's law the particle will start electrostatic repulsion. Thus coming to a closest distance ...
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Verify Conservation of energy for variable gravitational field [duplicate]

Remember we can verify conversation of energy in gravitational field when we take g to be constant When we leave body from H At a point h As potential energy is mgH -mgh Kinetic energy. ...
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Verify conservation of energy in gravitational field [on hold]

Remember we can verify conversation of energy in gravitational field when we take $g$ to be constant When we leave body from $H$ At a point $h$ As potential energy is $mgH -mgh$ Kinetic energy $=mv^...
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Why are there two photons in pair production Feynman diagram? [duplicate]

Given I wonder why are there two photons entering in a) pair production? Isn't it one photon resulting in $e^+e^-$ ?
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How to find the most likely decays in 2 hadrons for $\Omega$?

I took a look at the Particle Data Group, and the most likely modes are $\Omega ^{-}\rightarrow \Lambda K^{-}$ (68%) and $\Omega ^{-}\rightarrow\Xi ^{0}\pi ^{-}$ (24%). I have 2 questions: 1) The ...
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Why is this lagrangian term a surface term?

I've seen certain models add the term $J^\mu\partial_\mu \phi$ to a Lagrangian, where $J^\mu$ is the current for a $U(1)$ global charge. They always claim that, if the current is conserved $\partial_\...
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How do we predict the motion of this systems using Newton's laws of Motion?

This question should be seen as an argument for these Answers. I want you to read the answers to this following question before answering it. Violating Newtons First Law! Here Everybody is saying CM ...
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Find coefficient of friction [closed]

A plank P and a block Q are arranged by keeping block Q on the extreme left of plank P. The surface between P and Q has friction whereas coefficient of friction between plank P and ground is zero. ...
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Work energy Work energy theorem is only applied to particle [closed]

Work energy theorem is only applied to particles. 2) Work energy theorem should include both internal and external works. 3) The change in the system should be restricted to only kinetic energy
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Symmetries and conservation laws for a falling cat

For Hamiltonian systems, symmetries and conservation laws are defined and related to each other by Noether's theorem. Symmetry means the invariance of the Hamiltonian for a transformation group, and ...
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A man with a rifle shoots bullets at a speed [closed]

Hello all So I was doing this question. I had a doubt that, initially as both the (man and rifle ) system and the bullet were at rest we put 0 instead of m1u1 + m2u2 in equation of conservation of ...
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Energy and canonical momentum conservation in non-local classical field theory

Assume we have the following Lagrangian field density where $x, x'$ both three dimensional real vectors are coordinates and $t$ represents time, field is given by $\phi$. Assume for the sake of ...
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Does the law of conservation of momentum mean both linear and angular momentum?

I was solving a problem where a bullet hit a rod hinged about one of its ends. The rod is standing vertically before the collision. We had to find the final angular velocity of the rod. The way we did ...
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With respect to what does a Gyroscope maintain its orientation?

Assume ideal conditions, no friction, no energy loss in any form. Just an ideal gyro, in ideal conditions, spun into rotation and left alone. And placed somewhere on earth. From what I've read so ...
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Using conserved quantities to violate the Uncertainty Principle

If I have a two-body system (particles A and B) which I measure the total momentum of, can I measure the position of particle A very accurately and the momentum of particle B very accureately, and ...
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Underdetermined Problem with “Spring Launchers”?

Consider the following preliminary problem. A box of mass $m$ is attached to a box of mass $M$ by a compressed spring, and both are moving at velocity $v_{0}$. The compressed spring has potential ...
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Differing results from different conservation laws

I am currently studying for the GRE Physics subject test by working through published past tests. My question is about problem 44 from the test GR8677: $44.$ A uniform stick of length $L$ and ...
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Conservation of momentum — arm hitting object

Suppose I strike an object with my hand and I stop moving my hand as soon as I make contact with the object. This sends the object moving in a straight line with some velocity. Part 1: Is it valid to ...
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Does an evaporating black hole violate conservation of angular momentum?

Angular momentum is supposed to be conserved, but when a rotating black hole evaporates the Hawking radiation comes out in straight lines. Doesn't this violate conservation of angular momentum? Does ...
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What magnetic interaction makes spin not conserved?

If $[S_i,H]\neq0$ or $[S^2,H]\neq0$, we might say spin is not a good quantum number in the system. But is there any more practical or detailed criterion? Or certain families of magnetic interaction ...
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Calculating the exhaust velocity using conservation of momentum [closed]

I was just wondering if i did this correctly $$p_{exhaust}=-p_{rocket}$$ $$m_{e}v_{e}=-m_{r}v_{r}$$ $$v_{e}=\frac{-m_{r}v_{r}}{m_{e}}$$ I used data from a Saturn V simulation by Robert A. Braeunig ...
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A question about a small block sliding down a larger one, involving conservation of momentum and energy [closed]

I have been working on a problem for some time but haven't found a solution yet. I am mainly looking for a hint that will help me find a breakthrough and not a full solution. Here is the statement of ...
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Why are 2 gamma photons created? [duplicate]

When a positron and an electron come together they annihilate and produce 2 gamma photons $$e^+ +e^- \rightarrow 2\gamma$$ I can understand that they must be produced in pairs to conserve momentum. ...
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Spin Sums & Conservation of Angular Momentum

External fermions and bosons have momentum and spin (polarization) degrees of freedom. E.g. decay rate of $t\rightarrow W^{+}+\bar{b}$ with an unpolarized t beam. Peskin & Schröder sum over t, $\...
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Lavoisier law of mass conservation vs. conservation of energy

Lavoisier law states that for any isolated system, mass must be conserved over time. Also, Einstein's equation $E = mc²$ shows energy-mass equivalency. So, since mass must be conserved and energy ...
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What causes a rotating object to rotate forever without external force—inertia, or something else?

Someone told me that it is not inertia, but I think it is inertia, because it will rotate forever. In my understanding, inertia is the constant motion of an object without external force. Am I wrong?
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Chemical reactions in fluids

The equations of motion of ordinary relativistic hydrodynamics are derived by considering conservation equations of the energy tensor and particle number current, where constitutive relations relate ...
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Conservation of energy when external force is applied [duplicate]

I have heard that an isolated system is where no energy and no mass is exchanged with outside the system. Does that imply that no external force can be applied to an isolated system? Why? For example,...
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Conservation laws and Gauge transformations

I am studying gauge transformations, and my professor asked me: "Can the potentials obtained by the Lorenz gauge be considered physical quantities?" I assumed that "physical quantity" is ...
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The impact and collision principle

A particle P of mass 3m is moving in a straight line with speed u on a smooth horizontal table , a second particle Q of mass 2m is moving with speed 2u in the opposite direction of P on the same ...
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Energy Loss in Inelastic Collision

If we take a tennis ball and drop it from a certain height above the ground on earth, it collides inelastically, with the maximum height it can reach reducing after each collision due to loss of ...
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Newton's third law in magnetic in magnetic fields

Say I have a charged particle moving through a magnetic field perpendicular to it. It will experience a force, but according to Newton third law Every force has an equal and opposite reaction. So ...
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Symmetry modulo total derivative term in Noether's Theorem

I came across the proof of Noether's Theorem in David Tong's notes (page 14) on QFT. He writes something like, We say that the transformation $$\delta\phi(x) = \chi (\phi) \tag{1.34}$$ is a ...
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How is angular momentum conserved for the orbiting body if the centripetal force disappears? [duplicate]

When the centripetal force on an orbiting body disappears (e.g. if it the body is a ball and the force was exerted by a string and the string rips, or, more unrealistically, if the body is the earth ...
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Why is the velocity in the horizontal direction constant for 2D projectile motion?

For 2D projectile motion, why is the velocity in the horizontal direction constant? I learned that we neglect air resistance, but why? And isn't there horizontal acceleration at the beginning when ...
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Generalized symmetries

I am trying to teach myself about generalized symmetries. I have been attempting to read https://arxiv.org/abs/1412.5148, but am getting a bit confused. I'm hoping that maybe someone could explain the ...
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Does throwing a penny at a train stop the train?

If I stand in front of a train and throw a penny at it, the penny will bounce back at me. For the penny to reverse its direction, at some point its velocity must go to zero. This is the point it hits ...
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Collision of particles [closed]

I read in a book that in both elastic and inelastic collision, the interacting forces after the collision become effecitively zero, the potential energy of the system remains the same both before and ...
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How to prove the law of equal action and reaction?

Suppose $F=ma$ was known. For a system of two massive point objects, $a$ and $b$, suppose that $a$ act a force on $b$, $F_{ab}$, prove that $b$ also act a force on $a$, $F_{ba}$, and that $F_{ab}=-F_{...
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Why definition of potential energy and law of conservation of mechanical energy is misleading several times?

I regularly see 1 or 2 questions on this website about the definition or application of potential energy.The users fundamentally ask the same thing in every question. What I have learned till now is:-...
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Why conserve angular momentum about COM

In many questions involving collisions between Rigid bodies angular momentum is conserved about center of mass If bodies stick together after collision they estimate com and then conserve about ...
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Linear momentum of a rotating system

So say we have a thin rod resting horizontally on a flat frictionless surface. The rod is pinned at its center, and a small mass collides perfectly inelastically with one end of the rod after moving ...
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How to know the number of constants of a free particle?

Landau-Lifshitz Mechanics says that there are $2s-1$ constants of a system with $s$ degrees of freedom (beginning of the second chapter on Conservation Laws). If this is true, for a single free ...
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Does the conservation of $\frac{\partial L}{\partial\dot{q}_i}$ necessarily require $q_i$ to be cyclic?

If a generalized coordinate $q_i$ is cyclic, the conjugate momentum $p_i=\frac{\partial L}{\partial\dot{q}_i}$ is conserved. Is the converse also true? To state more explicitly, if a conjugate ...