# An elastic collision

1. Oct 2, 2013

### A. Aspart

A ball (mass m1 = 2 kg, velocity v = 5 m/s [the ball moves horizontally]) collides with another ball (mass m2 = 8 kg) which is hanging on a string (length l = 1.35 m). The collision is completely elastic. What is the tension force affecting the ball (m2) immediately after the collision?

I have a hard time understanding how to solve this since it’s an elastic collision. Aren’t momentum and kinetic energy conserved in an elastic collision? If so, wouldn’t I need to know the velocity of the smaller ball after the collision in order to solve the problem since the velocity of the smaller ball after the collision can’t be zero or the kinetic energy wouldn’t be conserved?

All help is much appreciated.

2. Oct 2, 2013

### nasu

You don't need the speed of m1 to solve the problem. But it does have a speed, sure.
Using conservation of momentum and of kinetic energy you can find the two speeds after collision.
To calculate the tension you only need the speed after collision for the ball attached to the string.