Physics quiz question: work on constant speed sled

Physics 205A Quiz 4, Fall Semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 6.2

[Version 1]
A Physics 205A student drags a sled across the horizontal floor at a constant speed by a rope that is inclined at angle of 20° above the horizontal. What is the sign of the work done by the normal force on the sled?
(A) Negative.
(B) Positive.
(C) (Zero.)
(D) (Not enough information is given.)

Correct answer: (C)

The normal force on the sled is perpendicular to the horizontal displacement, and thus the work it does on the sled is zero.

Student responses
Sections 70854, 70855
(A) : 2 students
(B) : 10 students
(C) : 11 students
(D) : 0 students

[Version 2]
A Physics 205A student drags a sled across the horizontal floor at a constant speed by a rope that is inclined at angle of 20° above the horizontal. What is the sign of the work done by the weight force on the sled?
(A) Negative.
(B) Positive.
(C) (Zero.)
(D) (Not enough information is given.)

Correct answer: (C)

The normal force on the sled is perpendicular to the horizontal displacement, and thus the work it does on the sled is zero.

Student responses
Sections 70854, 70855
(A) : 9 students
(B) : 9 students
(C) : 12 students
(D) : 0 students

Success level: 47%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.53