Happy and Sad Balls

Two balls are rolled down an incline, one at a time, and strike a wooden block that is standing upright. The Happy ball is a regular bouncy rubber ball, which rebounds from the block with a speed close to what it had before impact. The second Sad ball stops dead on impact. One ball knocks the block over - which is it?

  1. The Happy (bouncy) ball
  2. The Sad (non-bouncy) ball

The Sad ball experiences a change in momentum of -mv, going from mv to zero, so it must impart a momentum of mv to the block. The momentum of the Happy ball goes from +mv to -mv, a change of -2mv. It gives 2mv of momentum to the block, twice as much as the Sad ball.