Consider a system of two colliding bodies. If there is to be a collision, then at least one of the bodies must be moving, so the system has a certain kinetic energy and a certain linear momentum before the collision. During the collision, the kinetic energy and linear momentum of each body are changed by the impulse from the other body. For the rest of this chapter we shall discuss these changes-and also the changes in the kinetic energy and linear momentum of the system as a.whole without knowing the details of the impulses that determine the changes. The discussion will be limited to collisions in systems that are closed (no mass enters or leaves them) and isolated (no net external forces act on the bodies within the system).