Contact of Viscoelastic Spheres
  In a recent paper an implicit equation for contacting viscoelastic spheres was derived (Brilliantov et al. PRE 53, 5382 (1996)). Integrating this equation it can be shown that the coefficient of normal restitution depends on the impact velocity as 1-\epsilon\sim g^{1/5}. 
Thomas Schwager and Thorsten Poeschel