# Homework Help: Calculating Final Speed in Elastic collision (momentum)

1. May 22, 2012

### Mushroom79

1. The problem statement, all variables and given/known data

If it is eg. two balls, both going to the right before and after an elastic shock, how do you determine their velocities after the collision? (speeds before and weights are given)

Eg.

M = 50 kg
u1 = 20 m / s

m = 100 kg
v1 = 10 m / s

(M, m = mass)
(u1, v1 = velocity before the collision)
(u2, v2 = velocity after collision)

2. Relevant equations

The law of conservation of momentum:
m*v1+M*u1= m+M*v2*u2

Momentum before = Momentum after

3. The attempt at a solution

m*v1+M*u1= m+M*v2*u2 →

100 * 10 +50 * 20 = 100 +50 * v2 * u2

How do I continue from here?

Last edited: May 22, 2012
2. May 22, 2012

### ZxcvbnM2000

In elastic collisions energy is also conserved , so you can set up one more equation .

3. May 22, 2012

### Mushroom79

Oh, right. Think "momentum before equals momentum after" is what you meant.
Forgot to put it there.

4. May 22, 2012

### ZxcvbnM2000

In elastic collisions :

Momentum is conserved

Energy Is conserved

In inelastic collisions:

Momentum Is conserved

Energy is not conserved

Just to be more clear :P