In physics, an inertial frame of reference's a reference frame, tied to the state of motion of an observer, with the property that each physical law portrays itself in the same form in every inertial frame. The contrasting case's the set of non-inertial frames, in which the laws of physics change from frame to frame, and the usual forces governing laws of physics must be supplemented by fictitious forces. For example, Newton's second law for a particle in an inertial frame takes the form: » old… (More on Inertial)