FRAME OF REFERENCE Various physical phenomena, events, etc., require position and time coordinates to specify their complete description. Since nothing seems stationary in the universe, some arbitrary reference systems have to be defined with respect to which the position and the time coordinates for the event, etc., can be assigned. Such a system is called a 'frame of reference'. INERTIAL AND NON-INERTIAL REFERENCE FRAMES Newton's first law tells us that we can find a reference frame relative to which a body remains at rest or in uniform motion along a straight line when no net external force acts upon it. Such a reference frame is called an 'inertial reference frame'. Thus an inertial frame is one in which Newton's first law correctly describes the motion of a body not acted upon by a net force. Such frames are either at rest or moving with uniform velocity with respect to distant stars. Thus, they are unaccelerated and non-rotating. A reference frame attach...