Consider the swinging-balls apparatus. If two balls are lifted and released, momentum is c...

Consider the swinging-balls apparatus. If two balls are lifted and released, momentum is conserved as two balls pop out the other side with the same speed as the released balls at impact. But momentum would also be conserved if one ball popped out at twice the speed. Can you explain why this never happens?

(Hint: If the collision is perfectly elastic, what besides momentum would have to be conserved? Can you see why this exercise is here rather than in the previous chapter on momentum?)