  It is shown that the force in relativistic mechanics is not only the cause of
acceleration of particle relative to an inertial frame of reference, but also
the cause of change of the course of time along the particle's trajectory.
Therein lies the physical content of the dynamical principle underlying the
special theory of relativity (relativistic mechanics). The general formula for
the relative course of time between the points lying on the trajectory of
motion of particle under the action of a force field in an inertial reference
frame is derived. The applications of the theory developed to homogeneous
fields - to the field of gravity and electromagnetic field, and also to the
gravitational field produced by a point mass particle are considered. Physical
properties of the state of imponderability of particle in an external force
field are investigated. It is noted that the change in the course of time in a
force field is in no way connected with the change in space-time metric and is
a direct consequence of the causality principle of relativistic mechanics.
