Physics clicker questions: torque sequence

Physics 5A, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Multiple-Choice Question 8.6

Students were asked the following clicker questions (Classroom Performance System, einstruction.com) near the end of their learning cycle:

[0.6 participation points.] Which one of the forces in the figure produces the largest magnitude torque about the rotation axis indicated? (All forces have the same magnitude.)
(A) F_A.
(B) F_B.
(C) F_C.
(D) F_D.
(E) (More than one of the forces listed above has the largest magnitude torque.)
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 2 students
(B) : 21 students
(C) : 7 students
(D) : 5 students
(E) : 4 students
(F) : 0 students

Correct answer: (B)

Force A has zero torque, as it is parallel to its lever arm. All of the remaining forces are perpendicular to their lever arms. Force C and force D provide some torque, but force B has the greatest torque, due to having the longest lever arm.

[0.6 participation points.] How many of the forces in the figure above produces a positive torque about the rotation axis indicated? _____.

Sections 0906, 0907
0 : 1 student
1 : 29 students
2 : 8 students
3 : 1 student
4 : 0 students

Correct answer: 1.

Force B and force C would turn the square clockwise, and thus exert negative torques. Force D would turn the square counterclockwise, and thus would exert a positive torque.