In this verification case, a frictionless spherical particle falls freely under
gravitational force from an initial height, 
, and bounces upon collision with a rigid wall. The translation motion of the
particle is described in three stages: free fall, contact and rebound. For details regarding
the theory, refer to (Chen, 2007) and to Rocky DEM Technical Manual.
In this case, R is the ball particle radius, is the initial height, g is the gravitational
acceleration, 
is the pre-collision particle velocity and 
is the post-collision particle velocity, as described in Figure 2.15: Free-falling sphere colliding with a plane and frictionless surface..
