sir my question is ......why we used to calculate relative velocity...................please sir explain with example..........

We perceive motion when something is moving with respect to some other thing.

 

When we say that inside Earth, we are not moving because we cannot see anything moving with respect to Earth. But, if we see Earth from space, then we would see that it is moving with respect to stationary Sun.

 

Similarly, when a train is moving (assume: Its not shaking), the passengers inside will never come to know that they are moving. Only when a train passes this train do they come to know that their train is moving.

 

Hence, relative velocity is a must to calculate in real life scenarios.