Physics quiz question: net force on elevator

Physics 205A Quiz 3, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 4.25, 4.26

A 600 kg elevator attached to a cable moves downwards, with a decreasing speed. The direction of the net force on the elevator is:
(A) downwards.
(B) (has no direction, as the net force is zero.)
(C) upwards.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

The decreasing speed of the elevator means that Newton's second law applies to the motion of the elevator, such that the net force must be non-zero. Since the elevator is moving downwards as its speed is decreasing, its acceleration must be in the direction opposite its motion, and thus must point upwards. From Newton's second law, the net force and acceleration must have the same direction:

ΣF_y = m·a_y,

and since the acceleration points upwards, then the net force on the elevator must also point upwards.

Sections 70854, 70855, 73320
Exam code: quiz03eL3v
(A) : 40 students
(B) : 3 students
(C) : 27 students
(D) : 0 students

Success level: 39%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.44