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A numerical method is given for the solution of a system of ordinary differential equations and algebraic, unilateral constraints. The equations govern the motion of a mechanical system of rigid bodies, where contacts between the bodies are created and disappear in the time interval of interest. The ordinary differential equations are discretized by linear multistep methods. In order to satisfy the constraints, a quadratic programming problem is solved at each time step. The fact that the variation of the objective function is small from step to step is utilized to save computing time. A discrete friction model, based on Coulomb’s law of friction and suitable for efficient computation, is proposed for planar problems where dry friction cannot be neglected. The normal forces and the friction forces are the optimal solution to a quadratic programming problem. The methods are tested on four model problems. A data structure and …