# Homework Help: Two physics problems involving collisions

1. Jan 17, 2008

### Cowtipper

1) A 10.0-g object moving to the right at 20 cm/s makes an elastic head-on collision with a 15.0-g object moving in the opposite direction at 30.0 cm/s. Find the velocity of each object after collision.

2) A billiard ball rolling across a table at 1.50 m/s makes a head on elastic collision with an identical ball. Find the speed of each ball after the collision (a) when the second ball is initially at rest (b) when the second ball is moving toward the first at a speed of 1.00 m/s and (c) when the second ball is moving away from the first at a speed of 1.00 m/s.

I have no idea where to start...

Thanks for the help, I really appreciate it.

2. Jan 17, 2008

### hage567

What's conservation of linear momentum? What else is conserved in an elastic collision? You must know something.

3. Jan 17, 2008

### Cowtipper

Momentum and kinetic energy are conserved.

Conservation of momentum:

mvi1 + mvi2 = mvf1 + mvf2

I know that's what you have to use but I'm not too sure how...

4. Jan 17, 2008

### hage567

Note that the masses are different so it should be something like

m1vi1 + m2vi2 = m1vf1 + m2vf2

This is one equation you will need. Using the conservation of kinetic energy, set up another one. This will give you two equations and two unknowns (vf1 and vf2) and you will be able to solve for them.