M.6.13 Happy and Unhappy Impact

Two identical looking balls are suspended as pendula. One at a time they are lifted up to swing against a standing block. One ball easily knocks the standing block over, but the other does not. There is another pair of balls so you can show that the "happy" ball is elastic and bounces high, whereas the "unhappy" ball is totally inelastic and does not bounce. (F. Bucheit, Physics Teacher 23, 28, 1994).