  We study multiple elastic collisions of a block and a ball against a rigid
wall in one dimension. The complete trajectory of the block is solved as an
analytic function of time. Near the turning point of the block the force
carried by the ball is proportional to 1/x^3, where x is the distance between
the wall and the block, in the limit that the block is sufficiently heavier
than the ball. This is a unique pure mechanical system that reveals dipole-like
repulsion.
