The motion of an object to be manipulated is determined by the forces applied to the object. During a collision, impulsive forces may dominate all other forces, and determine the ultimate success or failure of a task. More effective planning and control of manipulators should be possible if the impact process, including the effects of friction and elasticity, is better understood. This paper explores the planar impact of two objects, and develops simple graphical methods for predicting the mode of contact, the total impulse, and the resultant motions of the objects. In the special case of a perfectly plastic collision, the fundamental motion of the object-whether an angular acceleration will occur, and if so in what direction-is the same as predicted in earlier work on quasi-static pushing.