Train A is moving at 100 kmh–1 through a station. Train B is also travelling at 100 kmh–1 through the station but in the opposite direction to train A. What is the speed of a seated passenger on? (a) train A relative to an observer on the station? (b) train A relative to another seated passenger on the same train? (c) train B relative to a passenger on train A?

Explanation:

(a) An observer on the station has a speed of 0 km/h.  The speed of  a passenger on Train A is 100 km/h.  The relative speed is 100 km/h − 0 km/h = 100 km/h.

(b) The speed of both passengers is 100 km/h, in the same direction.  The relative speed is 100 km/h − 100 km/h = 0 km/h.

(c) The speed of both passengers is 100 km/h, in opposite directions.  The relative speed is 100 km/h − (-100 km/h) = 200 km/h.