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

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1answer
207 views

Mathematical question on Collisions [closed]

A 2.5kg ball travelling with a speed of 7.5m/s makes an elastic collision with another ball of ...
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1answer
656 views

Question on Impulse-Momentum theory

A 5kg stone kicked by a student undergoes a velocity change from 6m/s to 10m/s in 4 seconds; ---- What is the momentum change of the stone within the time interval? ---- What impulse ...
0
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1answer
106 views

Period of an Object in Periodic Motion

My attempt (if it matters): The initial period is given by $T_X = \frac{2\pi X}{v}$ for some $v$. The new period is given by $T_Y = \frac{2\pi Y}{v}$ for the same $v$. $Y = \frac{X}{2}$, so ...
8
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1answer
431 views

Effect of the tail of the cat in the falling cat problem

To explain why a falling cat can turn by 180 degree without external torque and without violation of the conservation of angular momentum, one usually models the cat as two cylinders as in ...
20
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5answers
1k views

Is the converse of Noether's first theorem true: Every conservation law has a symmetry?

Noether's (first) theorem states that any differentiable symmetry of the action of a physical system has a corresponding conservation law. Is the converse true: Any conservation law of a physical ...
6
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3answers
776 views

How does $F = \frac{ \Delta (mv)}{ \Delta t}$ equal $( m \frac { \Delta v}{ \Delta t} ) + ( v \frac { \Delta m}{ \Delta t} )$?

That's how it's framed in my Physics school-book. The question (or rather, the explanation) is that of the thrust of rockets and how the impulse is equal (with opposite signs) on the thrust-gases and ...
3
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0answers
106 views

Videos of changing the orientation of an astronaut in space

Kane, Headrick and Yatteau describe in their paper "Experimental investigation of an astronaut maneuvering scheme" possible maneuvers to change the orientation in space without external torque. Is ...
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3answers
1k views

How do the Planets and Sun get their initial rotation?

How do the Planets and Sun get their initial rotation? Why do Venus and Mercury rotate so slowly compared to other planets and why does Venus rotate in a different direction to Mercury, Earth and ...
2
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1answer
223 views

How to express the heat capacity in terms of heat?

The first law of thermodynamics divides the internal energy change into contributions of heat and work. $$\text dU=\omega_Q-\omega_W,$$ Here I chose the notation to emphasise that the two parts are ...
6
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3answers
4k views

Why can't a single photon produce an electron-positron pair?

In reading through old course material, I found the assignment (my translation): Show that a single photon cannot produce an electron-positron pair, but needs additional matter or light quanta. ...
3
votes
1answer
186 views

Any case of a particle seemingly decaying into copies of itself?

Is there any case reported that seems to resemble the following: there is a particle and at some moment, the particle seems to break down into two or more particles that are all identical to the ...
3
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1answer
535 views

Can we explain physical similarities between Black Scholes PDE and the Mass Balance PDE (e.g. Advection-Diffusion equation)?

Both the Black-Scholes PDE{*} and the Mass/Material Balance PDE have a similar mathematical form of the PDE which is evident from the fact that on change of variables from Black-Scholes PDE we derive ...
1
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1answer
287 views

Rocket drive and conservation of momentum

I am currently reading through some lecture notes of Physics 1 and in a chapter about the dynamics of the mass point, there is an example covering the rocket drive. Let $v$ be the velocity of the ...
1
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1answer
203 views

Entanglement and conservation

Is the following assertion sufficiently unique to merit a paper? Every absolute conservation law implies a corresponding form of entanglement, not just spin (angular momentum). Linear momentum ...
8
votes
4answers
1k views

On what basis do we trust Conservation of Energy?

I'm happy to accept and use conservation of energy when I'm solving problems at Uni, but I'm curious about it to. For all of my adult life, and most of my childhood I've been told this law must hold ...
3
votes
1answer
176 views

In a Sterling Engine, does heat from the hot side transfer to the cold side?

A Sterling Engine is a closed system. The "hot" side oscillates between higher temperature with higher pressure and lower temperature with lower pressure. Does Nature switch back and forth between ...
3
votes
3answers
785 views

Is there more energy in the collapse of a cavitation bubble than the energy required to create the bubble in the first place?

The following does not include all scientific details and parameters, only a common summary of "thoughts". What is scientifically wrong with this summary? When you take your beer and tap the top ...
5
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2answers
642 views

What's the importance of Noether's theorem in Physics

The Noether's theorem that I want to mention is the following: Noether's theorem. I know the importance of Noether's contribution to modern algebra. Can anyone write about Noether's theorem in ...
2
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3answers
475 views

Train crash: are these situations alike? [duplicate]

I was just wondering... I believe that if a car travelling 50 miles per hour crashes into a wall, the result should be the same as crashing to another car also travelling 50 miles per hour (but in the ...
3
votes
3answers
196 views

What barriers exist to prevent us from turning a baryon into a anti-baryon?

At present the only way we can produce anti-matter is through high powered collisions. New matter is created from the energy produced in these collisions and some of them are anti-matter particles ...
10
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1answer
463 views

Neutrino Oscillations and Conservation of Momentum

I would like to better understand how neutrino oscillations are consistent with conservation of momentum because I'm encountering some conceptual difficulties when thinking about it. I do have a ...
2
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1answer
379 views

momentum conservation question involving a rocket and a spaceship [closed]

With the engines off a space ship is cruising at a velocity of 230m.s It fires a rocket straight ahead at the enememy vessel. The mass of the rocket is 1300kg and the mas of the ship (not including ...
0
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1answer
264 views

M1 vs M2 on a frictionless surface

I was wondering, if I had a frictionless surface and I had an M1 that was 100kg and an M2 that was 1kg and I pushed each separately with an equal force F would they move at the same speed or am I ...
2
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2answers
886 views

Testing conservation laws experimentally

How conservation laws are tested experimentally independently from each other? what do I mean by that question? It seems that to test one conservation law experimentally, such as conservation of ...
5
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5answers
741 views

Does the stress-energy tensor contain the equations of motion?

Derivatives $\nabla_i T^{ik}=0$ of a stress-energy tensor of physical system express conservation laws. Whether contains a stress-energy tensor also the information on the equations of motion of ...
2
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2answers
142 views

Can the charge of particles spontaneously flip from positive to negative or vice versa?

I'm thinking of matter antimatter annihilation, are there reactions where normal matter converts to antimatter?
2
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1answer
2k views

Heat Flux and Spatial Temperature Gradients

The derivation for Heat Equation I am reading starts by stating Net change of heat inside $[x,x+\Delta x]$ = Net flux of heat across boundaries + Total heat generated inside $[x,x+\Delta x]$ and ...
2
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0answers
194 views

Does electron go through a forbidden state when annihilate with positron?

Let's consider an electron-positron pair with total spin equal to zero. When it annihilates it can not emit only one photon because it would have zero momentum and nonzero energy. The pair emits two ...
25
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4answers
8k views

Why can't energy be created or destroyed?

My physics instructor told the class, when lecturing about energy, that it can't be created or destroyed. Why is that? Is there a theory or scientific evidence that proves his statement true or ...
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3answers
1k views

Conservation of electric charge in Feynman diagram

Here is a Feynman diagram showing the mutual annihilation of a bound state electron positron pair into two photons: Is the electric charge conserved at the point A (or B)? What is the "charge" of ...
2
votes
2answers
396 views

Which symmetry is associated with conservation of flux?

Which symmetry is associated with conservation of flux (e.g., in electromagnetism)? For example, when working with Gauss's law in electromagnetism, net flux through an arbitrary volume element ...
9
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0answers
617 views

Gauge redundancies and global symmetries

It is often said that local (gauge) transformation is only redundancy of description of spin one massless particles, to make the number degrees of freedom from three to two. It is often said that ...
1
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2answers
546 views

Conservation of momentum leading to damage

What would be an intuitive way to damage objects in a physics game using impulses? Since momentum is conserved, so is impulse (the change in momentum for any two time periods) in a closed system. So ...
2
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3answers
890 views

Why do some satellites fall to Earth?

In another question How does Newtonian mechanics explain why orbiting objects do not fall to the object they are orbiting?, one can read an affirmative answer. They how do you explain satellites ...
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3answers
616 views

Energy non-conservation for time-dependent potentials

Written in a book I read that the "total energy is not preserved when the potential depends explicitly on time", i.e. $U=U(x,t)$. Is there any proof or explanation for this?
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2answers
213 views

The time component is $\gamma m c$, so shouldn't $E=mc$?

Basically, the book is Brian Cox's Why Does $E=mc^2$?: (And Why Should We Care?). I just finished Chapter 5, where we derived the spacetime momentum vector (energy-momentum four vector, as he ...
3
votes
1answer
670 views

Conservation of angular momentum for a nonrigid body

Question: The sun is not a rigid body but a hot ball of gas. The period of rotation varies from 37 days at the pole to 26 days at the equator. The mean radius of the sun is $7\times 10^8\text{ ...
9
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1answer
95 views

Global symmetry in string theory

It is often stated that in quantum gravity only charges coupled to gauge fields can be conserved. This is because of the no hair theorem. If a charge is coupled to a gauge field then when it falls ...
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1answer
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Probability current

Conservation of probability: Suppose a wavefunction has ${\partial \mathbb P \over \partial t} = -t f(x,t)$ and ${\partial j \over \partial x} = i f(x,t)$. How does it follow that ${\partial \mathbb P ...
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3answers
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Is the continuous transformation of energy from one form to another one free? Or it consumes some quantity of who knows what?

Sorry for the unclarity. I probably created some bias in your mind having tagged my question with energy-conservation and conservation-laws. I simply consider that, relatively independently of ...
27
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1answer
3k views

What conservation law corresponds to Lorentz boosts?

Noether's Theorem is used to related the invariance under certain continuous transformations to conserved currents. A common example is that translations in spacetime correspond to the conservation of ...
14
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3answers
1k views

Decay of massless particles

We don't normally consider the possibility that massless particles could undergo radioactive decay. There are elementary arguments that make it sound implausible. (A bunch of the following is ...
26
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5answers
2k views

Why does everything spin?

The origin of spin is some what a puzzle to me, everything spin from galaxies to planets to weather to electrons. Where has all the angular momentum come from? Why is it so natural? I was also ...
10
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4answers
2k views

Deriving Newton's Third Law from homogeneity of Space

I am following the first volume of the course of theoretical physics by Landau. So, whatever I say below mainly talks regarding the first 2 chapters of Landau and the approach of deriving Newton's ...
2
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2answers
420 views

How do you find conserved quantities for linear second order ODEs?

I have a differential equation of the form $ \frac{d^2 y}{dt^2} + f(t) \frac{dy}{dt} + g(t) y = 0 $ where $f$ and $g$ are known functions of time. Is there a systematic (or otherwise) way of ...
13
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3answers
2k views

Why does each celestial object spin on its own axis?

AFAIK all the celestial objects have a spin motion around its axis. What is the reason for this? If it must rotate by some theory, what decides it's direction and speed of rotation? Is there any ...
3
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1answer
436 views

Substance like quanties and conserved quantities, Karlsruhe physics course

In the Karlsruhe physics course one defines the term "substance-like" quantity: Let my cite the definition from a paper by Falk, Herrmann and Schmid: "There is a class of physical quantities whose ...
3
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4answers
1k views

Energy conservation in Electrodynamics

Let us suppose that we have a known electromagnetic wave-train of finite size propagating in a certain direction. There is a probe charge on its way. This EMW is an external field for the charge. The ...
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3answers
9k views

How does the 'water jet pack' work?

So I was cruising around at YouTube and saw this sweet video, and as I was watching started to wonder: "How is this possible?". For a little bit of background, in case you decide to not watch the ...
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4answers
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If all conserved quantities of a system are known, can they be explained by symmetries?

If a system has $N$ degrees of freedom (DOF) and therefore $N$ independent1 conserved quantities integrals of motion, can continuous symmetries with a total of $N$ parameters be found that deliver ...