# Tagged Questions

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

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### Why is $\int_{s} \mathbf{h}\cdot \mathbf{n} da = - \dfrac{dQ}{dt}$ & not $\int_{s} \mathbf{h} \cdot\mathbf{n} da = - \dfrac{dQ}{dt} .{dt}$?

I was reading the Lectures of Feynman about surface integral where a situation in which heat is conserved has been dealt. Let there be $Q$ heat energy present inside a body. Now, if there is net heat ...
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### Continuity equation in fluid mechanics

The continuity equation in fluid mechanics states that $$\frac{\partial\rho}{\partial t} + \nabla\cdot(ρ\mathbf u)=0$$ Can you explain to me what is the physical meaning of each term of the ...
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### What is the definition of parity conservation?

I searched quite hard, and am still confused what is the exact definition of parity conservation? For example, we have quantum system with initial state $\Phi_i$, and after decaying it comes to final ...
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### Proving the conservation of 4-momentum for a particle collision $A+B\to C+D$

Let me say that particle A hits particle B and two particles come out - C and D; In system S I can write: $$p_A^μ+p_B^μ=p_C^μ+p_D^μ;\tag{1}$$ here $p_N^μ$ is the 4-momentum. Using the Lorentz ...
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### Two balls travelling at different speeds collide in two referentials

In a referential R1, one ball B1 travels at 100 m/s and hits and another identitcal ball B2 that travels at 50 m/s in the same direction. Assuming the material in which the balls are made is such that ...
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### Calculation of velocity via kinetic energy and momentum yielding different answer

I am attacking the given problem (as a preface I'm not asking to be spoon fed any answers, just looking for clarity from people much smarter than myself) A 15.0kg block is attached to a very light ...
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### How is the conservation of momentum satisfied in long-range attraction such as electromagnetism and gravity?

I'm not a physicist, but my understanding is that electromagnetism (including attraction between opposite charges) is mediated by the photon, and gravity is probably (hypothetized to be?) mediated by ...
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### During perfectly elastic collision of two objects $A$ and $B$, how much is the initial speed of object $A$ affecting the force applied to $A$ itself?

I got stuck in what seemed to be an easy problem. If two bodies $A$ and $B$ collide perfectly elastically and head-on what is the equation that gives us the forces applied to $A$ and $B$, given a ...
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### Why is momentum conserved in inelastic collisions? How is it related to momentum-impulse theory?

First of all I want to mention, that I've found many questions around this site and in other websites dealing with my question; however, I don't think they answer my question fully. So I am here to re-...
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### The momentum of a swinging sword

Suppose you are faced with a zombie, and the only way to kill it and save yourself is to chop its head off with your sword. However, you are very weak from illness, and can only afford to strike once. ...
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### How is Angular Momentum Conserved when Mass is Released?

I am not a physicist (math/comp-sci) but I understand that Angular Momentum is supposed to be conserved. I find this confusing because there seems to be many simple, common cases where a restrained, ...
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### Angular momentum in planetary disk formation

This question is actually more linked to astronomy and astrophysics than to pure physics. I tried posting it on the astronomy page, however it got no answers, so I though this page might help. ...
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### If there are 4 dimensions, shouldn't objects appear and disappear in 3D space?

If there are 4 (or more) physical dimensions, and physical objects can move through the 4th dimension in paths perpendicular to our 3 dimensions, the physical objects must pass through our 3-...
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### How did the planets (in the Solar System) start to revolve around the sun if they were attracted towards the Sun via the gravitational force? [duplicate]

The planets in the Solar System revolve around the Sun in almost circular paths called orbits. The Sun pulls the planets with the gravitational force,but the planets do not get drawn to the Sun but ...
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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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### Equations for a collision between two particles

Say I have two particles on a 2D plane, they have a x and y coordinate, a x and y velocity, a mass, a coefficient of restitution and a coefficient of friction. What formulae would I need to determine ...
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### How do we prove that the initial velocity is equal to final velocity relative to centre of mass?

In elastic collision it is stated that the initial velocity relative to centre of mass is equivalent to final velocity of centre of mass of the same object. How do we prove that?
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### How does the Earth rotate, given that the torque acting on it while revolving is zero?

I've come to understand that the torque acting on the Earth while revolving the Earth is zero. Torque is the force responsible for rotation of a body. So how does the Earth rotate?
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### Can conservation of momentum be violated?

The law of the conservation of momentum has been established for hundred of years. Even in Quantum field theory every particle collision must be momentum-conserving if there is homogenity in space. ...
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### How does frequency change with centripetal force?

Using the equation $$F_c = {{mv^2}\over{r}}$$ I can see that mass and velocity are directly proportional to centripetal force. I can also see that the radius length is inversely proportional to ...
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### Intuition as to why the orientation (of a 3D object) is not a conserved quantity?

Say you start off floating in space, in a fixed position and orientation, with zero linear and angular velocity, with no external forces. So you are a closed mechanical system. By twisting your body ...
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### Problem on electromagnetic induction and Newtonian physics

Please imagine a solenoidal toroid (i.e. a donut shaped inductor) powered by an AC voltage source. It creates a changing magnetic field which is confined to the interior of the toroid (i.e. within the ...
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### Why can't Saturn's rings be absorbed into the planet due to it's gravity?

What stops Saturn's rings from going into the planet?
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### Basic question about angular momentum

I've learned that the angular momentum of an object rotating about a fixed axis is $I \omega$. Also, in absence of external torques, $I_1 \omega_1 = I_2 \omega_2$ (meaning, two different events). I ...
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### How does a wheel balance itself during circular motion? [duplicate]

A wheel (or any ring of considerable mass) hardly balances itself when it is placed vertically on ground, but when we roll it along the ground it balances itself. What causes this effect? I guess its ...
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### Ballistic pendulum - are all forces conservative for this case?

For the Ballistic pendulum in the image below: Are we allowed to assume that after the bullet hit the log (mass M), then there's a conservation of energy? (Thus $\frac{1}{2}(M+m)V^2 = (M+m)gh$) ...
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### Elastic Collisions and Relative Velocities

In a 1D elastic collision, it is well-known that the relative velocities of the two objects (before and after the collision) are reversed. What is the extension of this result to 2D or higher? Is ...
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### Is momentum conserved in the collision of a ball with a hanging rod?

Suppose we have a situation like A ball of some mass $m$ with some velocity collides with rod hinged at point $A$. Is momentum conserved in this situation? I know that hinge will give impulsive ...
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### “Where” does dissipated enstrophy go?

We are all familiar with the kinetic energy dissipation and how it is converted into heat which can either be radiated away or go into the internal energy of the system. In the enstrophy transport ...
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### Energy and momentum conservation - why it is so fundamental?

Over hundreds of years the conservation of energy and momentum in a closed System was proven. 100 years ago, Emmy Noether showed that these fundamental laws arise from the following facts and vice ...
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### Relationship between energy density and energy flux

I'm presently working on obtaining conservation laws via symmetries. These conservation laws are written as 2-element vectors where each element is the energy density and energy flux. To proceed in my ...
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### Off-shell legs in Feynman diagrams

I have a tree-level diagram with one leg being off-shell (its momentum beeing $\mathcal{O}(m_B)$). How do I treat this leg when computing the amplitude? Do I put in the propagator and ignore the ...
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### Rolling on a frictionless pond

I have a doubt about one of the questions in my textbook. Q- You are standing with your bag in your hands in the middle of a friction-less pond. How can you come out of the ice? There is of ...
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### Separability of Hamilton Jacobi Equation

When we talk about integrability of classical systems in terms of Hamiltonian or Lagrangian mechanics, it's all to do with counting independent conserved quantities. Then when we move to the Hamilton-...
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### Why do planets not stop revolving around the Sun? [duplicate]

Why do planets revolving around the Sun not stop revolving? Note I am not asking why planets do not collapse with Sun.
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### When is it appropriate to drop pressure terms when applying conservation of momentum to a fluid?

I'm trying to wrap my head around pressure forces in incompressible, irrotational, invicid flow. Applying conservation of momentum to a control volume gives me \frac{\partial}{\...
I was told a proof that the electric field was conservative (without using $\nabla$) which used a point charge and showed the following: $$w.d.=\int_c{\vec F \cdot \mathrm{d} \vec l}=\int_c{\vec F\... 4answers 274 views ### Thermodynamics thought experiment There is some ideal gas in a container moving with some velocity on a smooth surface and you suddenly stop it( say by using your hands) , will the temperature of the gas increase? It seems to me that ... 2answers 383 views ### How to use the first law of thermodynamics for simple mechanical systems? I'm confused about what exactly is Q and U and their signs. Consider a block initially having some kinetic energy which we stop and we want to find by how much amount its temperature increases. ... 2answers 287 views ### Is it possible to determine the outcome of any impact knowing only the ratio of masses? [duplicate] In elastic collisions in 2-D if two balls A, B (m_A = m_B, R = 1) have equal mass we can determine in advance the outcome of the collision. If cue-ball A impacts object-ball B (at rest) ... 2answers 188 views ### Energy conservation in electrodynamic system Consider two charged particles initially at rest in the configuration below. Let us assume the following: Starting at time t=0, we apply a constant force f to the the bottom particle so that ... 3answers 235 views ### Is it in general true that \nabla_\mu T^{\mu\nu}=0 implies the matter equations of motion? I know of several cases where the covariant conservation of the energy momentum tensor \nabla_\mu T^{\mu\nu}=0 can be used to derive the equations of motion of the matter fields. Is this in general ... 1answer 428 views ### Conservation of angular momentum in a free rod When a collision is elastic and no external torque acts on a system, angular momentum is conserved I found this example and checked the results: A ball (m = 1 Kg , v = p =+22 m/s, Lm = +11, Ke = 242 ... 2answers 330 views ### Classical EM : clear link between gauge symmetry and charge conservation In the case of classical field theory, Noether's theorem ensures that for a given action$$S=\int \mathrm{d}^dx\,\mathcal{L}(\phi_\mu,\partial_\nu\phi_\mu,x^i)$$that stays invariant under the ... 1answer 134 views ### How to derive the conserved current of the Klein Gordon equation? Similarly to the probability current in non-relativistic quantum mechanics, there is a conserved current for the Klein Gordon equation, however a different one. I'm trying to calculate that. The KG ... 1answer 366 views ### How does the space shuttle move in a vacuum? [duplicate] If there is no friction in space, then how does the space shuttle boost? According to Newton's law we move by pushing something; in space, there is vacuum, so how the space shuttle moves without ... 5answers 479 views ### Physical interpretation of the statement \oint E\cdot dl=0 Can anyone provide me with a physical interpretation of \oint E\cdot d\ell=0 in electrostatics? 4answers 292 views ### Which is the right explanation for rocket motion? [duplicate] What actually causes a rocket to move? Is it the pressure in the rocket engine or the amount and velocity of mass that is being ejected out. The reason I am asking is, I found these two explanations ... 1answer 142 views ### Reaction in Particle Physics I'm currently checking some reactions if they are allowed. One of them is$$\nu_\mu p\to\mu^-\Delta^{++} Since this is a weak interaction I've checked the Baryon and Lepton number, Spin and the ...
I am trying to find the photon energies of the decay $\pi_0 \rightarrow \gamma\gamma$ and their dependence on the pion energy $E_{\pi}$, its initial velocity $\beta$ and the scattering angle between ...