The total kinetic energy is the sum of two terms, the first of which, Kcm, can never change: it is, in fact, as constant as the total momentum itself, since it involves the center of mass veloc...The total kinetic energy is the sum of two terms, the first of which, Kcm, can never change: it is, in fact, as constant as the total momentum itself, since it involves the center of mass velocity, vcm, which is proportional to the total momentum of the system (recall equation (3.3.4)).