Question

An athlete swims from the north end to the south end of a 50.0 m pool in 20.0 s and makes the return trip to the starting position in 22.0 s.
(a)What is the athlete’s average velocity for the first half of the swim?
(b) What is the average velocity of the athlete for the second half of the swim?
(c) What is the athlete’s average velocity for the round trip?

Answer 1

Take south as the positive direction (or state directions explicitly).

(a) Displacement = 50.0 m south in 20.0 s:
v_avg = 50.0/20.0 = 2.50 m/s south.

(b) Displacement = 50.0 m north in 22.0 s (i.e. −50.0 m if south is +):
v_avg = −50.0/22.0 = −2.27 m/s ⇒ 2.27 m/s north.

(c) Round trip displacement = 0 in total time 20.0 + 22.0 = 42.0 s:
v_avg = 0/42.0 = 0.00 m/s.

(For contrast, the average speed for the round trip = total distance/total time = 100.0/42.0 ≈ 2.38 m/s.)