The diagrams below show two cars moving along the same straight road.

The speeds are relative to the ground.

Measurements of position and speed are made from some point on the ground, for example point O.

Speed of car A relative to point O = v_A(O) = 40ms^-1

Speed of car B relative to point O = v_B(O) = 30ms^-1

What is the speed of car A relative to car B?

In one second, A moves 40m along the road but B only moves 30m along the road.

So in one second, A moves 10m away from B in the positive sense and B moves 10m away from A in the negative sense.

This simple example illustrates that, in general, to find the relative speed of two bodies subtract their speeds relative to a third body.

Consider a slightly more dangerous situation:

Suppose that we want to know the speed of car A relative to car B.