Two spheres are perfectly vertically alligned and falling towards the ground. They are separated by a small distance. The bigger sphere is on the bottom, and both spheres are supposed to have the common velocity v just before they strike the ground. I have to calculate the velocity of the smaller sphere, after the bigger sphere hits the ground. All collisions are perfectly elastic, and I am told to solve this as a sequence of closely spaced impulsive collisions. Help or ideas anybody? I am not sure what the sequence of collisions I am calculating are. And I am also not sure how to use either the lab or center of mass frame of references since the lab frame supposes that the velocity of one of the masses is zero which is not true, and the center of mass frame assumes the total momentum is zero, and I am not convinced that this is true.