ABSTRACT: The article attempts to show a concrete example, that the application of the special theory of relativity, when considering the motion of a closed mechanical system of bodies in inertial reference systems, can lead to the fact, that the momentum of a closed system will be a function of time.
KEYWORDS: The special theory of a relativity, the law of conservation of momentum of the closed mechanical system, dependence of weight of a body on speed of its movement, symmetry of space and time, relativity principle.