ABSTRACT
When a ball is placed on a horizontal surface which is vibrating sinusoidally in the vertical direction, it will loose contact with it as soon as the maximum acceleration of the plate is greater than the gravity g. It will perform a free flight and fall again on the plate, bouncing with a velocity that depends on the instantaneous velocity of the plate and the restitution coefficient ε. But in the case of a layer of granular material, the energy is quickly dissipated in the collisions among grains. The restitution coefficient is then very small, and the layer as a whole can be considered as an inelastic solid.
