Relative Velocity
 
 
 
The terms ‘rest’ and ‘motion’ are only relative. For example, when we say that a train is moving with velocity 30 m.p.h, we mean is that it is the velocity with which the train moves with respect to an observer on the earth who is regarded as fixed. This is not true strictly since a person on the unconsciously partakes the rotator motion of the earth round its axis and the motion of the earth round the sun. In addition, he shares the motion of entire solar system through space with respect to certain fixed stars. Thus there is no absolute fixed point on the earth about which we can measure motion. Hence a person on the earth can never realize absolute motion or absolute rest.
 
 
 
Let us consider two motor cars A and B moving in the same direction on a road with equal speed. To a person seated in A, if he were unconscious of his motion, the car B would appear to be at rest. The line joining the two cars will always remain constant in magnitude and direction.