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.