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An inertial frame of reference is a reference frame in which an observer is either at rest or moving with a constant velocity. In such a frame, an object that is not subjected to any net external forces will either remain at rest or continue to move in a straight line at a constant speed, according to Newton's first law of motion (the principle of inertia). Inertial frames are significant in classical mechanics because the laws of motion are consistently observed in these frames.