Abstract
We have studied the coefficient of restitution, η, in normal collisions of a non-rotating sphere on a massive plate for a range of materials, impact velocity and sphere size. The measured coefficient of restitution does not monotonically vary with velocity. This effect is attributed to dynamics that occur during the finite duration of impact, since the time scales of the vibrational modes of the plate are comparable to the contact time. The measured effect is robust and is expected to be ubiquitous in fluidized granular media. We also find that η is a decreasing function of sphere radius, with a dependence that is not captured by existing models of impact.