Motion of a body is always described with reference to some well defined coordinate system. This coordinate system is referred to as 'frame of reference'.& A frame of reference that remains at rest or moves with constant velocity with respect to other frames of reference is called inertial frame of reference (or inertial frame) . An inertial frame of reference is actually an non-accelerated frame of reference. Newton's laws of motion are valid in all inertial frames of reference. In this frame of reference a body does not acted upon by external forces. All inertial frames of reference are equivalent for the measurement of physical phenomena. Examples of inertial frames:
• Our earth.
• A space shuttle moving with constant velocity relative to the earth.