In mechanics, a means by which an observer specifies positions and describes the motion of bodies. For example, an observer may use a Cartesian coordinate system or a polar coordinate system. In some circumstances, it may be useful to consider two or more different frames of reference, each with its own observer. One frame of reference, its origin and axes, may be moving relative to another. The motion of a particle, for example, as it appears to one observer will be different from the motion as seen by the other observer.
A frame of reference in which Newton's laws of motion hold is called an inertial frame (of reference). Any frame of reference that is at rest or moving with constant velocity relative to an inertial frame is an inertial frame. A frame of reference that is accelerating or rotating with respect to an inertial frame is not an inertial frame.
A frame of reference fixed on the Earth is not an inertial frame because of the rotation of the Earth. However, such a frame of reference may be assumed to be an inertial frame in problems where the rotation of the Earth has little effect.