Hello Craig

There are two principles involved here. They aremomentumandrestitution. In the equations that follow, I am taking the direction as positive, and I'm assuming that the velocities of and are and respectively, after the impact. So:

1The momentum of the system is conserved at the impact.So (noting that the masses are all 1 kg):

(1)

2 The restitution equation is:the velocity of separation = -e times the velocity of approach.

In other words, the (velocity of velocity of ) after impact (velocity of velocity of ) before impact. So (remembering that we are taking as the positive direction):

(2)

Add (1) and (2):

and if collides with

Grandad