To model the bouncing ball system we assume that the table's mass is much greater than the ball's mass and the impact between the ball and the table is instantaneous. These assumptions are realistic for the experimental system described in the previous section and simply mean that the table's motion is not affected by the collisions. The collisions are usually inelastic ; that is, a little energy is lost at each impact. If no energy is lost then the collisions are called elastic . We will examine both cases in this book: the case in which energy is dissipated (dissipative) and the case in which energy is conserved (conservative) .