You are stationary by the road and observe two cars approaching you. A red car is moving in one lane with a speed 20 m/s relative to you. In a second lane, a blue car is moving at a speed 8 m/s with respect to the red car. How fast is the blue car moving with respect to you?
A) Must be 14 m/s.
B) Must be 22 m/s.
C) Must be 28 m/s.
D) Must be 12 m/s.
E) Could be either 28 m/s or 12 m/s.

Question

Quizplus · Accepted Answer

The blue car is moving at 8 m/s relative to the red car, and the red car is moving at 20 m/s relative to the observer. Therefore, the blue car is moving at 8 m/s + 20 m/s = 28 m/s relative to the observer. However, the blue car could also be moving at 8 m/s - 20 m/s = 12 m/s relative to the observer, if it is moving in the opposite direction of the red car.

You Are Stationary by the Road and Observe Two Cars