“…Moreover, the characteristic relaxation time 1/η is assumed to be much larger than the impact duration, that is, ηt c ≪ 1 or, which is asymptotically the same, ηt 0 c ≪ 1. The analytical approximations are given in terms of the small dimensionless parameter ε, which, in view of (1) and (24), is proportional to ηt 0 c , where t 0 c is the Hertzian impact duration (see Equations ( 1) and (7) 2 ). Namely, Hertz's theory of impact yields the following characteristic time:…”