Give an example to explain that motion is relative in nature

Consider a long train moving at speed v. Now consider a passenger throwing a ball inside this train, towards the back of the train, with same velocity v (but in the opposite direction of the train movement).

- A passenger inside the train will see the ball moving with speed v

- For an observer outside the train, however, the ball will appear as still. In fact, for him the ball will have a speed v (given by the movement of the train) - v (velocity of the ball but moving in the opposite direction), so the net velocity will be v + (-v) = 0.