Physics quiz question: tipping a square plate

Physics 205A Quiz 5, spring semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 8.1

A Physics 205A student would like to turn a heavy square plate clockwise around a pivot by pushing or pulling on it using different forces separately. The force on the square plate that would produce the largest magnitude torque is:
(A) F_A (applied horizontally at corner diagonal to the pivot).
(B) F_B (applied horizontally at corner above the pivot).
(C) F_C (applied upwards at corner left of pivot).
(D) (There is a tie.)
(E) (Not enough information is given to determine this.)

Correct answer (highlight to unhide): (D)

All three applied forces have the same magnitude. Also, they all have the same perpendicular lever arm ℓ, drawn from the pivot to intercept their respective lines of forces perpendicularly. Thus for each of the three forces, they have the same magnitude torque τ = ℓ·F.

Student responses
Sections 30880, 30881
(A) : 1 student
(B) : 13 students
(C) : 8 students
(D) : 17 students
(E) : 0 students

Success level: 43%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.46