On momentum conservation

The relativistic law of momentum transformation shows that the sum of momenta of even isolated particles is not invariable in all inertial reference systems. This is connected with the relativistic change of kinetic energy and mass of a system of particles in result of internal interactions. The paper proposes a short and simple proof on the necessity of potential momentum. The momentum conservation law (for all interactions in the Minkowski world) is expressed in a generalized form. The constancy of the sum of kinetic and potential momentum of closed system of particles is shown. The energy conservation is a necessary condition. The potential momentum is defined as usual (e.g. as in the Berkeley Physics Course). (author). 13 refs