I'm not sure how to do this. The Problem: We have two balls one with mass M and another with mass m. M>>>>>m both balls are dropped at the same time, the heavier ball below the lighter ball from a height of 10 meters. (even though the lighter is on top and heavier on bottom, for the purposes of this problem, you can assume they are falling from the same height). So both balls fall. When the heavier ball hits the ground, it bounces straight back up and immediately collides with the lighter ball falling down. THe question is how high will the lighter ball rise assuming all collisions are elastic. My Take: Both balls start at a potential energy. Bigger ball has M*g*10m and smaller ball has m*g*10m. For big ball, PE turns into KE so we have: Mgh = .5*M*v^2 so velocity when it hits the ground is 14.14m/s (g = 10 m/s^2) which is the velocity with which it would hit smaller ball. THe smaller ball would hit the ground at the same velocity but i don't know how to figure out what it's velocity is going to be after it collides with the bigger ball. I'm thinking something involving conservation of momentum to find that velocity and then just figure out max height of a ball of mass "m" with that initial velocity... Please help.