The Hertz contact in chain elastic collisions
  A theoretical analysis about the influence of the Hertz elastic contact on a three body chain collision is presented. In spite of the elastic character of the collision, the final velocity of each particle depends on the particular interaction between them. A system involving two elastic spheres falling together one in top of the other under the action of gravity, and colliding with an horizontal hard wall is studied in detail. The effect of the Hertz contact interaction can be easily put in evidence for some particular situations. 
P. Patricio