Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.22
Students were asked the following clicker questions (Classroom Performance System, einstruction.com) in the middle of their learning cycle:
A 2,010 kg elevator moves with an upward acceleration of 1.50 m/s^2.
Which free-body diagram shows the tension and weight forces acting on the elevator, and the resulting net force?
(N.b.: (F) is the "I'm lost, and don't know how to answer this" response.)
Sections 30880, 30881
(A) : 25 students
(B) : 7 students
(C) : 6 students
(D) : 2 students
(E) : 3 students
(F) : 0 students
This question was asked again after displaying the tallied results with the lack of consensus, with the following results. No comments were made by the instructor, in order to see if students were going to be able to discuss and determine the correct answer among themselves.
Sections 30880, 30881
(A) : 39 students
(B) : 0 students
(C) : 0 students
(D) : 1 student
(E) : 0 students
(F) : 0 students
Correct answer: (A)
The elevator is accelerating upwards, thus net force must point upwards as well, from Newton's second law. Thus FDBs (B) and (E) are eliminated for having downwards net forces, and FBD (C) is eliminated for having a zero net force. FBD (D) has the downwards weight force more than the upwards tension force, which is not consistent with its upwards net force, and thus be eliminated for being internally inconsistent. (Similarly, FBD (E) is also "impossible.")
FBD (A) correctly shows an upwards net force, and the corresponding upwards net force as having a greater magnitude than the downwards weight force.
Pre- to post- peer-interaction gains:
pre-interaction correct = 58%
post-interaction correct = 98%
Hake, or normalized gain
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