Consider a classical inelastic collision of the form a + b -> c+ d. (for example, this could be a collision such as na + cl -» nat + cl in which two neutral atoms exchange an electron and become oppositely charged ions.) show that the law of conservation of classical momentum is invariant under the galilean transformation if and only if total mass is conserved- as is certainly true in classical mechanics. (we shall find in relativity that the classical definition of momentum has to be modified and that total mass is not conserved.)