  This paper proposes a gravitodynamic theory because there are similarities
between gravitational theory and electrodynamics. Based on Einstein's principle
of equivalence, two coordinate conditions are proposed into the
four-dimensional line element and transformations. As a consequence,the
equation of motion for gravitational force or inertial force has a form similar
to the equation of Lorentz force on a charge in electrodynamics. The inertial
forces in auniformly rotating system are calculated, which show that the
Coriolis force is produced by a magnetic-type gravitational field. We have also
calculated the Sagnac effect due to the rotation. These experimental facts
strongly support our proposed coordinate conditions. In addition, the
gravitodynamic field equations are briefly discussed. Since only four
gravitational potentials (3 + 1 split) enter the metric tensor, the
gravitodynamic field equations in ``3+1 split" form would be analogous to
Maxwell's equations.
