Question

Amy is standing still on the ground; Bill is riding his bicycle at 5 m/s eastward; and Carlos is driving his car at 15 m/s westward. How fast does Bill see Carlos moving and in what direction?

Answer 1

Answer:

20 m/s westward

Explanation:

Taking eastward as positive direction, we have:

$v_B = +5 m/s$ is the velocity of Bill with respect to Amy (which is stationary)

$v_C = -15 m/s$ is the velocity of Carlos with respect to Amy

Bill is moving 5 m/s eastward compared to Amy at rest, so the velocity of Bill's reference frame is

$v_B = +5 m/s$

Therefore, Carlos velocity in Bill's reference frame will be

$v_C' = v_C - v_B = -15 m/s - (+5 m/s) = -20 m/s$

and the direction will be westward (negative sign).