Q. What are inertial and non-inertial frame of references?
(i) Inertial (or) unaccelerated frames:
Bodies in this frame follow Newton's law of intertia as well as other laws of Newtonian mechanics. In this frame a body stays at rest or in continuous motion unless acted upon by an external force.
(ii) Non-inertial (or) accelerated frames:
A frame of reference is supposed to be a non-intertial frame when a body not acted upon by an external force is accelerated. In this frame Newton's laws aren't valid.