By contrast, a non-inertial reference frame has non-zero acceleration. In such a frame, the interactions between physical objects vary depending on the acceleration of that frame with respect to an inertial frame. Viewed from the perspective of classical mechanics and special relativity, the 'usual' physical forces caused by the interaction of objects have to be supplemented by fictitious forces caused by inertia. Viewed from the perspective of general relativity theory, the inertial (i.e. fictitious) forces are attributed to geodesic motion in spacetime.