Posted by **Lucy X** on Tuesday, April 28, 2009 at 10:12pm.

A 0.25-kg ball is attached to a 26-cm 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 head-on collision with a 0.21-kg 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 0.21-kg ball just after the collision? (Answer: 2.5 m/s)

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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

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