Frame of Reference

How can we know that the object is at rest or in motion?

In order to answer this question, we shall consider the following illustration.

Suppose a bag is lying in a train moving with a constant speed. A passenger in the train would find the bag stationary but a person standing by the roadside would find the bag in motion. Thus, the bag is at rest if it is observed from the train, it is in motion if it is observed from the roadside. If we observe the trees, building etc. from the ground, they are stationary but from the train they will be found to be in motion. Thus, motion is a combined property of the object under study and the observer. In other words, motion is a relative concept.

A place and a situation from where an observer takes his observation is called frame of reference.

In the above illustration of the bag, the frame of reference for the passenger is the train moving with constant speed, while for the observer standing by the roadside the frame of reference is the stationary, Earth. 

The frames of reference is of two types:

· Inertial reference frame

· Non-inertial reference frame