Relative velocity is the velocity that one object has when viewed from another moving object. It is something that we are frequently aware of throughout the day, although we are not thinking in scientific terms. For example, if you are sitting still and a dog walks past, it seems to be moving quite quickly. If you later go for a run and pass the dog, still moving at the same velocity, it will seem to be travelling much more slowly.
When you are traveling in a car or bus or train, you see the trees, buildings and many other things outside going backwards. But are they really going backwards? No, you know it pretty well that it’s your vehicle that is moving while the trees are stationary on the ground. But then why do the trees appear to be moving backwards? Also the co-passengers with you who are moving appear stationary to you despite moving.
It’s because in your frame both you and your co-passengers are moving together. Which means there is no relative velocity between you and the passengers? Whereas the trees are stationary while you are moving. Therefore trees are moving at some relative velocity with respect to you and the other passenger. And that relative velocity is the difference of velocities between you and the tree. The relative velocity is the velocity of an object or observer B in the rest frame of another object or the observer A.
Picture Credit : Google
