  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.
