An inertial reference frame is one in which Newton's first and second laws of motion are valid.

Hence, within the inertial frame, an object or body accelerates only when a physical force is applied, and (following Newton's first law of motion), in the absence of a net force, a body at rest will remain at rest and a body in motion will continue to move uniformly?ie. in a straight line and at constant speed.

 

 

 non inertial frame of reference is one in which a body violates Newton's Laws of Motion, mainly the First Law. In such a frame, despite no real force acting on a body at rest, it might move; or one that was already moving come at rest or change it's direction of motion. For comparison see an inertial frame.

Newton's first and second laws of motion do not hold in non-inertial reference frames. Specifically, masses in non-inertial reference frames appear to feel fictitious forces (such as the Coriolis force or the centrifugal force) that derive from the acceleration of the reference frame itself. Fictitious forces cause apparent accelerations in objects without any physical force causing the acceleration. Fictitious forces are proportional to the mass upon which they act; if such forces are observed, scientists will recognize that they are in a non-inertial reference frame. For example, the rotation of the Earth can be observed from the Coriolis force acting on a Foucault pendulum.

An apparent exception would seem to be the force of gravity, which is also proportional to the mass upon which it acts. Although gravity can be considered a "real" physical force for the purposes of calculations in classical mechanics, Albert Einstein showed in his theory of general relativity that gravity itself can also be considered a fictitious force. In his theory, the free-falling reference frame is equivalent to an inertial reference frame (the equivalence principle). By contrast, Einstein noted that observers standing on the Earth are experiencing an unrecognized acceleration from the normal force pushing up on their feet and, thus, are in a non-inertial (accelerated) reference frame. Further details may be found under general relativity.