  In this paper, we point out that the 4-vector force acting on a particle is
always in the direction orthogonal to the 4-vector velocity of the particle in
the 4-dimensional space-time, rather than along the line joining the particle
and the action source. This inference is obviously supported from the fact that
the magnitude of the 4-vector velocity is a constant. This orthogonality brings
out many new aspects for force concept. In this paper it is found that the
Maxwell's equations can be derived from classical Coulomb's force and the
orthogonality, some gravitational effects such as the perihelion advance of
planet can also be explained in terms of the orthogonality.
