Change of Mass in the Conservative System of Bodies
  A problem of mass in macro- and microcosm has been considered from the single point of view on the basis of the law of conservation of energy. It is shown that in the conservative (absolutely closed) system all types of motion and interaction of the bodies are possible only at the expense of the mass defect of bodies themselves. As the result, their mass becomes less than the initial rest mass before forming the system. It leads to some change in the energy balance of the system and it is noticeably manifested in the motion of bodies in the relativistic velocity region. The accounting of that essentially adds a customary picture for the motion of bodies in the conservative system. 
A.V. Glushkov