A frame of reference for measurements of motion, space, and time. An inertial frame (also called a Newtonian frame or system) is one in which a body subject to no net forces moves in a rectilinear path with constant velocity. In Newtonian physics, distances measured in two such systems will be related by the ‘Galilean transformation’, x′=x + vt, where x, x′ are the two measures, t is the common time, and v is the relative velocity. The Michelson-Morley experiment showed that this was not true for the speed of light, which is a constant for all observers. See also clock paradox, relativity.