When an object is at rest on the table, it is not really at rest because the table cannot be at rest due to the motion of the earth. The earth undergoes numerous types of motions such as rotation, revolution of the earth, rotation of the solar system, movements of the galaxies and the expansion of the universe etc.

In other words, the term ‘rest’ is meaningless. To give a meaning to the term ‘rest’ a boundary MUST be defined. This boundary is known as the Frame of Reference (Frame of Inertia).

The motion of an object must be studied relative to a frame of reference. If the object doesn’t change the position relative to the boundary, the object is considered to be at rest.

For example, consider a passenger who is sitting in a moving bus. An observer A, who is there inside the bus, observes no motion of the passenger. Hence, for the observer A, the bus can be considered as the frame of reference.

An observer B, who is there standing outside the bus, observes the same passenger, moving with the same speed of the bus. For observer B, the frame of reference can be taken as the road. Hence, relative to the road, the passenger is moving with the same speed of the bus.