Physics midterm question: average speed versus magnitude of average velocity

Physics 205A Midterm 1, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 2.7

The braking distance(*) for a 2012 Chrysler 300C to slow down from 31 m/s to a complete stop is 50.3 m. Assume that the acceleration is constant during this process. As the car slows down to a stop, which is the greater quantity?
(A) Average speed.
(B) Magnitude of average velocity.
(C) (There is a tie.)
(D) (Not enough information is given.)

(*) "Braking, 70–0 mph: 165 ft," Andrew Wendler, "2012 Chrysler 300C Long-Term Road Test Update," http://www.caranddriver.com/reviews/2012-chrysler-300c-long-term-update-review.

Correct answer: (C)

Since the car always moves in the forward direction in a straight line without reversing direction, then the distance traveled will be identical to the displacement for this time interval. Then the average speed (distance traveled divided by elapsed time) would then be equal to the magnitude of the average velocity (magnitude of displacement, divided by elapsed time).

Sections 70854, 70855
Exam code: midterm01sWFf
(A) : 5 students
(B) : 11 students
(C) : 39 students
(D) : 1 student

Success level: 68%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.69