# Homework Help: Conservation of Momentum

1. Nov 9, 2006

### lonelywizard

Can someone help me to prove the law of conservation of momentum using an example?

2. Nov 10, 2006

### QuantumCrash

Hmm... right now I can't really think of any that actually PROVES it. But I can prove that it always works no matter what situation it is applied to. Just show a situation, e.g. car accidents, rockets, springs even.

3. Nov 11, 2006

### Andrew Mason

The proof is one line:

$$F = dp/dt = 0$$

That is the definition of force. If the force acting is 0, dp/dt = 0 so p is constant.

AM

4. Nov 11, 2006

### QuantumCrash

I don't actually thinks that actually proves it. That just defines force. Other than practical experiments, or actually showing it works in every situations, I can't really think WHY it must be so.

Perhaps it is derived from elastic collisions, and kinetic energy is conserved. Perhaps not.

5. Nov 11, 2006

### Andrew Mason

Conservation of momentum follows from Newton's first law. Since:

$$F = dp/dt$$

IF:

$$F = 0$$

then: P = constant.

Why is Newton's first law true? It may not be. But to prove something, youi have to start with a premise. The premise is that F = dp/dt, Newton's first law. So far, no one has been able to show that it is not true.

AM

Last edited: Nov 11, 2006
6. Nov 11, 2006

### physics girl phd

Ohhh... the problem of asking to "prove" versus "verify/demonstrate". :uhh:

7. Nov 12, 2006

### QuantumCrash

Thats proof by contradiction that is. Since you can't disprove it that means its right. I suppose you might accept it that way. But what I am going about is more that you can't proof by induction and you can't really explain WHY momentum is conserved.