# Homework Help: Momentum and Impulse

1. Jul 12, 2010

### mizzy

1. The problem statement, all variables and given/known data
A locomotive of mass 21x10^3kg travelling at 2.0m/s runs into an identical stationary locomotive.

If the collision is perfectly elastic and lasts for 1.5s, find the average force exerted on the stationary locomotive during the collision.

2. Relevant equations
Find momentum before and after the collision (m1v2 + m2v2 = m1v2 + m2v2)

Use impluse to find the average force (F = p/t)

3. The attempt at a solution
I found the momentum before and after the collision.

For the second part, i'm not sure which momentum to use. The final one?

Can someone tell me if I'm right?

2. Jul 12, 2010

### Staff: Mentor

That should be F = Δp/t.

You need the change in momentum.

3. Jul 12, 2010

### mizzy

the change in momentum after the collision?

4. Jul 12, 2010

### Staff: Mentor

Change = Final - Initial

I guess the initial momentum of the stationary locomotive was zero; so you're OK with using the final momentum.

5. Jul 12, 2010

Thanks