Before discussing relative velocity we must know that all the motion is relative. Everything moves-even things that appear to be at rest. They move relative to the Sun and stars. When we discuss the motion of something, we describe the motion relative to something else. When we say a racing car reaches a speed of 200 kilometers per hour, we mean relative to the track. Unless stated otherwise, when we discuss the speeds of things in our environment, we mean relative to the surface of Earth. Motion is relative. Now we come to relative velocity Before studying further I am assuming that you are familiar with the concept of frame of reference and velocity.
Observations made in different frames of reference are related to each other to know how, consider this example of two trains approaching one another, each with a speed of 90 km/h with respect to the Earth. Observers on the Earth beside the train tracks will measure 90 km/hr for the speed of each of the trains. Observers on either one of the trains (a different frame of reference) will measure a speed of 180 km/h for the train approaching them. When the velocities are along the same line, simple addition or subtraction is sufficient to obtain the relative velocity. But if they are not along the same line, we must make use of vector addition. And it is also necessary that when specifying a velocity, we specify what the reference frame is.