[en] In their formulation of the classical equations of motion to describe heavy-ion collisions, D. Gross and his colleagues introduced radial and tangential friction. The source of these terms is the transfer of mass, energy, linear and angular momentum which occurs during the course of the collision. The imaginary term in the complex potential of the optical model for nucleon-nucleus elastic scattering describes an absorption which has two causes. There is of course the absorption which is present because of the open channels which give rise to reactions such as the inelastic scattering. In addition, there is the absorption because the optical model describes the prompt amplitude. The flux which is delayed is not included in the optical-model amplitude. Theoretically the prompt amplitude is obtained from the exact amplitude by energy averaging. Both of these mechanisms lead to energy deposited in the target nucleus and should therefore be described by friction. (orig.)