1. The problem statement, all variables and given/known data A 90-g ball moving at 100 cm/s collides head-on with a stationary 10-g ball. Determine the speed of each after impact if (a) they stick together, (b) the collision is perfectly elastic. 2. Relevant equations p=mv 3. The attempt at a solution I already solved A, but I don't know how to do B. So far, I've gotten: p=mv = 90(100) = 9000 g.cm/s p=mv = 10 (0) = 0 so... p1+p2=p3 m1 v1 + m2 v2 = (m1 +m2)v 90(100) + 0 = 100v v= 90 cm/s I don't understand how the answer can be 80 cm/s when they perfectly elastic. Shouldn't it be 90 cm/s?