# Questions tagged [conservation-laws]

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

2,386 questions
Filter by
Sorted by
Tagged with
24k views

### Why does Newton's Third Law actually work?

My father explained to me how rockets work and he told me that Newton's Third Law of motion worked here. I asked him why it works and he didn't answer. I have wasted over a week thinking about this ...
15k views

### Why do electrons, according to my textbook, exist forever?

Does that mean that electrons are infinitely stable? The neutrinos of the three leptons are also listed as having a mean lifespan of infinity.
17k views

### What conservation law corresponds to Lorentz boosts?

Noether's Theorem is used to relate the invariance of the action under certain continuous transformations to conserved currents. A common example is that translations in spacetime correspond to the ...
20k views

### Why does a billiard ball stop when it hits another billiard ball head on?

(I'm repeating myself a lot here, but it's because I want to make my confusion clear.) If 2 billiard balls are the same exact mass, and one hits another stationary one head on, I have heard that the ...
5k views

### Why do travelling waves continue after amplitude sum = 0?

My professor asked an interesting question at the end of the last class, but I can't figure out the answer. The question is this (recalled from memory): There are two travelling wave pulses moving in ...
21k views

### 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 ...
7k views

### Is there a way for an astronaut to rotate?

We know that if an imaginary astronaut is in the intergalactic (no external forces) and has an initial velocity zero, then he has is no way to change the position of his center of mass. The law of ...
33k 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 ...
7k 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 ...
10k 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 ...
4k views

### In substances where light is slowed down, is its momentum lessened and if so, how does it speed up again when emerging from that substance? [duplicate]

I think in some media, light can be significantly slowed down; but even if only slightly, where would the momentum go when the light slows down and where does it get the extra momentum when it leaves ...
194k views

### How can momentum but not energy be conserved in an inelastic collision?

In inelastic collisions, kinetic energy changes, so the velocities of the objects also change. So how is momentum conserved in inelastic collisions?
9k views

### Why does the speed of a ping pong ball increase when the space in which it can bounce decreases?

I was playing table tennis the other day when I my ball fell off the table. I placed my paddle above it in order to slow it down, and then I brought the paddle to the ground so that the ball would ...
9k views

### Can Noether's theorem be understood intuitively?

Noether's theorem is one of those surprisingly clear results of mathematical calculations, for which I am inclined to think that some kind of intuitive understanding should or must be possible. ...
8k views

### Why don't helicopters use reaction wheels to counter the main rotor?

As the main title says. I'm finding myself wondering about helicopters. The tail rotor is a vulnerable and key piece of equipment, especially on military helicopters. I know some helicopters instead ...
21k views

### Newton's 3rd law… hitting drywall (which I break) vs hitting a brick (which breaks me)?

According to the Third Newton's law of motion: For every action there is an equal and opposite reaction. So, I understand that if I hit a brick wall with $50\, \mathrm{lbs}$ of force, the brick ...
19k views

### When you hit a baseball, does the ball ever travel faster than the bat?

It seems impossible, yet I'm thinking that maybe because the ball compresses against the bat a bit it acts a little like a spring, and DOES travel faster than the bat? EDIT: This is just a ...
8k views

### Do all the conservation laws of Physics take no time to propagate?

For instance, conservation of momentum, does it take time to propagate between two or more objects? If it does, then there would be some moment that the momentum is not conserved. If it doesn't take ...
2k views

### 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 ...
16k views

10k views

### Why don’t astronauts “push” spacecraft?

Perhaps it goes without saying, but according to Newton’s laws “every action has an equal and opposite reaction”. How do astronauts, especially those inside small spacecraft like the Crew Dragon, not “...
5k views

### Is it possible to conserve the total kinetic energy of a system, but not its momentum?

It is possible to conserve momentum without conserving kinetic energy, as in inelastic collisions. Is it possible to conserve the total kinetic energy of a system, but not its momentum? How? To ...
8k views

### Are there any fluids that flow slower in a constricted region, unlike water?

It is well known that when water flows through a tube you can make it flow faster by making the tube narrow. Now consider what happens when a group of people are moving and the space becomes narrower....
6k views

### 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?
901 views

### Best current bounds on nonconservation of momentum?

It's not straightforward to test conservation of momentum experimentally, and many experiments that seem like tests really aren't. For example, in a Newtonian system of identical particles that ...
11k views

### Can a car steer on a frictionless surface?

Do the front tires of a car act like gyroscopes, such that a car could steer on a frictionless surface?
19k views

### Is it possible to shoot bullets in space or would the recoil of the gun be too strong?

I've read a few articles that say that astronauts have already brought guns in space and that shooting bullets in space is possible. But won't the recoil of the gun be too strong? Law of ...
4k views

### How can there be net linear momentum in a static electromagnetic field (not propagating)?

I understand from basic conservation of energy and momentum considerations, it is clear in classical electrodynamics that the fields should be able to have energy and momentum. This leads to the usual ...
7k views

### Why does shooting a handgun accelerate a bullet to deadly speed without injuring the gun user's hand?

Momentum is defined by the product of mass and velocity. Now a projectile out of a gun has to have high velocity to penetrate a human body, as its mass isn't significant. But to reach this velocity, ...
7k views

### Which specific law of physics is broken by the “double jump”? [closed]

So, there's a cliche in computer gaming known as a "double jump". This can be described as: A character jumps from a solid surface, and then is able to extend their jump by carrying out the jump ...
4k views

### Why didn't the debris collapse back into the Earth at the time of Moon's formation?

The most accurate theory for the formation of our moon is the Giant impact hypothesis, which says that a Mars sized body collided with our early Earth and after this collision all the debris got ...
1k views

### What happens to the quantum information of a particle and an antiparticle when they annihilate?

I understand that the quantum no-deleting theorem dictates that it's impossible to delete quantum information, so what happens to the quantum information of a particle and an antiparticle when they ...
3k views

### No hair theorem for black holes and the baryon number

The no hair theorem says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge. For me it is puzzling why ...
5k 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 ...
24k views

### Is it possible to walk in a friction-less world?

Is it possible to walk in a frictionless world. I think you won't be able to walk because walking depends on friction acting on your feet. I think about it like an ice rink because when you try to ...
5k views

### 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 ...
2k 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 ...
24k 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. My ...
9k views

### Noether's current expression in Peskin and Schroeder

In the second chapter of Peskin and Schroeder, An Introduction to Quantum Field Theory, it is said that the action is invariant if the Lagrangian density changes by a four-divergence. But if we ...
1k views

### 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 ...
5k views

### What is the conserved quantity of a scale-invariant universe?

Consider that we have a system described by a wavefunction $\psi(x)$. We then make an exact copy of the system, and anything associated with it, (including the inner cogs and gears of the elementary ...