Inertial and Non Inertial Frames

To study motion in one dimension (in a straight line) a reference point (origin) is enough. But, when it comes to motions in two and three dimensions, you have to use a set of reference lines to specify the position of a point in space. This set of lines is called frame of reference.

Every motion is described by an observer. The description of motion will change with the change in the state of motion of the observer. For example, consider a box lying on a railway platform. A person standing on the platform will say that the box is at rest. A person in a train moving with a uniform velocity v will say that the box is moving with velocity –v.

A frame of reference is fixed with the observer to describe motion. If the frame is stationary or moving with a constant velocity with respect to the object under study (another frame of reference), then in this frame law of inertia holds good. Therefore, such frames are called inertial frames.

On the other hand, if the observer’s frame is accelerating, then it is called non-inertial frame. For the motion of a body of mass m in a non-inertial frame, having acceleration (a), you can apply second law of motion by involving a psuedo force ma. In a rotating body, this force is called centrifugal force.