Question

A car travelling with a velocity of 90km/hr and other with 72km/hr in opposite directions, find the relative velocity between them.

Answer 1

Let us change the units to m/s first.

Velocity of bus A ,  

Velocity of bus B,  

For the relative velocity, understand that since the buses are moving in the opposite direction, their velocities will add up. Why? Have you ever sat in a car and looked at the cars travelling towards you? They seem faster than they actually are. We are actually utilising the same concept here, that since the buses are travelling towards each other, both are contributing in changing the distance between the buses in the same manner (either increasing or decreasing, but not both).

For finding the displacement, assuming they started at the same initial position, the displacement is

Answer 2

relative velocity is 18 km/hr in the direction of car which have speed 90km/hr.

When two objects is moving in different direction then subtract smaller speed from larger speed and the resultant relative velocity will be in the direction of bigger speed