Physics
posted by Lucy X .
A 0.25kg ball is attached to a 26cm piece of string. The ball is first raised so that the string is taut and horizontal, then the ball is released so that, at the bottom of its swing, it undergoes an elastic headon collision with a 0.21kg ball that is free to roll along a horizontal table.
a) What is the speed of the swinging ball just before the collision? (Answer: 2.3 m/s)
b) What is the speed of the 0.21kg ball just after the collision? (Answer: 2.5 m/s)

It's an elastic collision, so I know that total energy and momentum are conserved.
I think I need to write two equations (one for kinetic energy and one for momentum), but I don't know how... :S

a gyroscope consist of a rotating disk with a 48.7cm radius suitably mounted at the mid point