Problem 60.

While you are traveling in a car on a straight road at 90 km/hr, another car passes you in the same direction; its speedometer reads 120km/hr. What is your velocity relative to the other driver? What is the other car's velocity relative to you?

Solution:

The relative velocity is equal to the difference of velocities.

So if you need to find your relative velocity then you need to subtract from your velocity the velocity of another car:

The minus sign here gives us the direction of relative velocity: the direction is opposite to the directions of motions of the cars.

Similarly we can find:

The plus sign here gives us the direction of relative velocity: the direction is the same as the directions of motions of the cars.