20121013

Physics midterm question: in a downwards elevator, slowing down

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

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 4.11

Which free-body diagram (A)-(F) represents a Physics 205A student in an elevator that is slowing down while descending? Clearly circle your answer below.

For a object moving downwards, but slowing down in speed, the acceleration and net force must point in the direction opposite its velocity, which would be upwards. This eliminates responses (A), (B), and (C), which would have downwards net forces, and responses (D) and (F), which have zero net forces. There are only two forces acting on the elevator (normal force from the floor of the elevator, which is upwards; weight, which is downwards), and the magnitude of the upwards normal force is greater than the downwards weight force.

Sections 70854, 70855
Exam code: midterm01sWFf
(A) : 0 students
(B) : 0 student
(C) : 20 students
(D) : 4 students
(E) : 32 students
(F) : 1 student

Success level: 56%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.48