The observed reduction in acceleration of a mass at higher velocities compared with that at lower velocities, for apparently the same applied (driving) force, in not due to increase in inertia with velocity, as relativity theory suggests. It is due to a reduction in effective force with velocity. The force balance (Newton's third law) is secured by the inertia of the energy left in the intermediate region. Mass remains an invariant, as does the ratio of force to the time derivative of the momentum (Newton's second law). It is only necessary to use the correct proper times for each part of the interacting systems, as well as the hereto belonging proper velocity numbers. Proper times result from the actual physical nature of time dilatation (clock retardation). Forces, velocities, and time are interlinked physical quantities and will attain special unique numerical values only in the frame that is at absolute rest, the Mach frame. Inertia appears linked to magnetic microfields between the moving charges that always accompany matter. Such microfields can be considered analogous to those representing chemical binding energy. Symmetry requirements will then determine the straight path of inertial movement, and when violated, cause the centrifugal effects. Practical equations do not change because of the new nonrelativistic interpretation of the interactions, but certain paradoxes of the presently reigning theories are resolved.