1. (1) We have, above, assigned an intelligible meaning to this expression if the body be considered as a member of a system. But, taken absolutely, it has no meaning.

2. (2) If our shot, now considered separately, meet directly a target of mass M′ and there be inelastic collision, and if the relative velocity be U, then the velocity u′ of the shot relatively to the c.m. of this new system will be , and the velocity V′ of the target relatively to this c.m. will be .

The total internal energy of this system as measured by the heat given out will be , and of this the shot may with some reason be said to contribute the first term, and the target the second, in virtue of its K.E. [see VII., (3) and (4)].