Physics quiz question: dragged book, with friction

Physics 205A Quiz 3, fall semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 4.77

[Version 1]
Consider a book on a horizontal table, with a tension force of 10.0 N acting on it. The magnitude of the normal force of the table on the book is 22.0 N. The coefficient of static friction is 0.42. The coefficient of kinetic friction is 0.35. Which force has the greatest magnitude?
(A) Friction force on the book.
(B) Weight of the book.
(C) Tension force on the book.
(D) (Not enough information is given to determine this.)

Correct answer: (B)

The maximum static friction force magnitude is mu_s*N = 9.2 N, where the normal force is 22.0 N. The kinetic friction force magnitude is mu_k*N = 7.7 N. Since the tension force (10.0 N) is greater than the maximum static friction force magnitude, then the book is unstuck, and is sliding and accelerating to the right. From applying Newton's first law in the vertical direction, the weight of the book is 22.0 N. Thus the weight of the book has the greatest magnitude, while the (kinetic) friction force has the least magnitude.

Student responses
Sections 70854, 70855
(A) : 3 students
(B) : 12 students
(C) : 7 students
(D) : 0 students

[Version 2]
Consider a book on a horizontal table, with a tension force of 10.0 N acting on it. The magnitude of the normal force of the table on the book is 22.0 N. The coefficient of static friction is 0.42. The coefficient of kinetic friction is 0.35. Which force has the least magnitude?
(A) Friction force on the book.
(B) Weight of the book.
(C) Tension force on the book.
(D) (Not enough information is given to determine this.)

Correct answer: (A)

Student responses
Sections 70854, 70855
(A) : 17 students
(B) : 2 students
(C) : 4 students
(D) : 0 students

Success level: 64%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.45