So let's say you're standing on a linoleum floor and you have a basketball and a golf ball. You drop the golf ball from a height of 6 feet and because of the conservation of momentum, it bounces back up a distance X. You do the same with the basketball and it bounces back up a distance of Y. So then you make the assumption that if you drop the two balls together with the golf ball sitting on top of the basketball, the two momentums will add together and the golf ball will bounce back a distance of X + Y. But in actuality, the distance bounced is far greater. Why does this happen?