This paper presents new results in the cooperative manipulation of objects by two or more manipulators. The case of contact due to additional manipulators grasping, or regrasping, a common object held by one or more manipulators is discussed. A model of instantaneous contact is derived, and the impulse force generated by contact found, based on the dynamic models of the manipulators and object. Applications to planning and control of interactive manipulation of common objects by multiple arms are discussed.