  We outline two subjects of relativistic mechanics: (i) the set of allowable
world lines, and (ii) the origin of the relativistic law of dynamics governing
point particles. We show that: (i) allowable world lines in the classical
theory of particles and fields are quite simple geometric objects as opposed to
their associated three-dimensional trajectories, and (ii) Newton's second law
requires neither modification nor generalization, it should be only smoothly
embedded in the four-dimensional geometry of Minkowski spacetime to yield the
dynamical law for relativistic particles.
