# Homework Help: Elastic radial collision?

1. Oct 26, 2008

### emmi1987

1. The problem statement, all variables and given/known data
A ball is attached to one end of a wire, the other end being fastened to the ceiling. The wire is held horizontal and the ball is released from rest. It swings downward and strikes a block initially at rest on a horizontal frictionless surface. Air resistance is negligible and the collision is elastic. The masses of the ball and block are, respectively, 1.6 kg and 2.4 kg and the length of the wire is 1.2 m. Find the velocity (magnitude and direction) of the ball just before the collision and just after the collision.

... I don't even know where to start with this one, I can't find anything about radial? (not sure if that's the appropriate term) motion in regards to velocity. Any help would really be appreciated.

2. Relevant equations

3. The attempt at a solution

2. Oct 27, 2008

### tiny-tim

Hi emmi1987!

Radial is towards/away from the centre.

Tangential is round the circle (and tangential velocity = angular velocity times radius).

But you don't need either … just use conservation of energy for the first part, and the usual elastic collision equations for the second part.

3. Oct 27, 2008

### emmi1987

Thanks! I'm golden on the second part.. but for the first part, do i need to find the time it takes for the ball to fall in order to find v? and then use conservation of energy?

4. Oct 28, 2008

### tiny-tim

Hi emmi1987!

No, the time doesn't matter …

conservation of energy doesn't depend on how you get from state A to state B (state = position + speed), or how long it takes …

KE(A) + PE(A) = KE(B) +PE(B).