# Conservation momentum in 2D

1. Oct 29, 2008

### HelloMotto

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 headon collision with a 0.21 -kg ball that is free to roll along a horizontal table.

What is the speed of the 0.21 kg ball just after the collision?

2. Oct 29, 2008

### olgranpappy

first figure out the speed of the 0.25kg ball right before the collision.

3. Oct 29, 2008

### HelloMotto

ok i got like 2.26 m/s just before the collision.
i know that in perfectly elastic collision, the kinetic energy is conserved.
so my energy statement would be something like this
Ek initial = Ek ball1 + Ek ball2. right?

but when i expand and plug in the givens, Im left with 2 unknowns, the final velocity of each ball. Thats where Im stuck at.

4. Oct 30, 2008

### olgranpappy

the solution to your problem is given as the title to this thread... use conservation of momentum.