A hard ball of mass M subjected to the gravitational force Mg
bounces off of the floor at z=0. The potential energy experienced
by the ball is sketched in the figure below. Note that the ball
cannot go through the floor to regions where z<0. Due to the
uncertainty principle, even in its minimum energy state, the ball can
never be consistently at rest at z=0. Instead, the ball always has
some characteristic non-zero kinetic energy T>0 and height 
above the ground.
