1. The problem statement, all variables and given/known data Say you have a big ball of mass m1 and a little ball on top of that of mass m2 (assume they are a small distance apart, like 1mm). Now lets drop these from a height of h so that the big ball will bounce off the ground and collide into the little ball in an elastic collision. 2. Relevant equations p=mv 3. The attempt at a solution I know gravity would play a key role in this example but how would you perform calculations with it? I know F=p/t and momentum will not be conserved since there is an external force (gravity) so knowing this, how can you determine how high each ball will rise after the collision? I was thinking of turning the force of gravity into momentum by multiplying with time but that is an unknown variable.