The capacity of a system for doing work by virtue of the velocities of the various moving masses in the system. The kinetic energy (k. E. ) of a body of mass M moving with velocity v is ½ M v 2. It is equal to the work that would have to be expended to bring the body to rest. It is additive, in the sense that the k. E. of a system composed of several moving bodies is the sum of the k. E. S of the individual bodies. A distinction is sometimes made between a body's internal k. E. and its external or mechanical k. E. The internal k. E. is the sum of the k. E. 's of the individual particles making up the body, with respect to a coordinate system fixed in the body. The external k. E. is the k. E. appearing as translation and rotation of the body as a whole in some external coordinate system. Unless stated otherwise, kinetic energy means external kinetic energy.