Define relative velocity of one body with respect to another.

Explanation

If `vec(v_(1)) and vec(v_(2))` are the velocities of two bodies 1 and 2, then, relative velocity of first body with respect to the second body is `vecv=vec(v_(1))-vec(v_(2))` for both travelling in the same direction and `vecv=vec(v_(1))+vec(v_(2))` when the two bodies are travelling in the opposite directions.