  The impact of a two-dimensional elastic disk with a wall is numerically
studied. It is clarified that the coefficient of restitution (COR) decreases
with the impact velocity. The result is not consistent with the recent
quasi-static theory of inelastic collisions even for very slow impact. The
abrupt drop of COR has been found due to the plastic deformation of the disk,
which is assisted by the initial internal motion. (to be published in
Proceedings of 9 th Nisshin Engineering Particle Technology International
Symposium on 'Solids Flow Mechanics and Their Applications' held at Kyoto,
7th-9th January, 2001
