Résumé
The mutual impenetrability of the parts of a mechanical system and their possible confinement by given external obstacles are unilateral constraints, the description of which results in joint inequalities arid equalities. The system dynarrùcs is thus governed by a set of very nonsmooth relations. The effect of dry friction at possible contacts mak:es evolution problems still less regular; velocity jumps may occur in this case, even in the absence of collision. In developing time-discretization procedures to solve these evolution problems numerically, the authors have chosen to face nonsmoothness without resorting to any regularization technique.