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When two robot arms execute coordinated motions in assembly, collision between the two end-effectors cannot be avoided. The collision effects on the motions of two coordinating robots are studied in this paper. At the moment of a collision, an impulse exerted on the two end-effectors is generated. This impulse will cause abrupt velocity-changes in robot motions. The direction of the impulse depends on the relative orientation of the end-effectors when the collision takes place. The magnitude of the impulse, on the other hand, depends on the relative velocity between the two end-effectors immediately before the collision, the joint positions, and the inertia matrices of the two robots. The minimization of the abrupt velocity-changes is discussed. The study reveals that to reduce the abrupt velocity-changes, the two coordinating robots must be symmetrical in all aspects, and the direction vector of the impulse must be an eigenvector of the Jacobian inertia matrix for the robot arms. Since the two conditions impose constraints on the robot trajectories, the approach of minimization is valid only when the two robot arms follow pre-planned paths.