Physics quiz question: book dragged at constant speed

Physics 5A Quiz 3, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

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

[3.0 points.] A 4.0 kg book is horizontally pulled by a string at constant speed along a horizontal table. The coefficient of kinetic friction is 0.35. What is the magnitude of the net force acting on the book?
(A) 0 N.
(B) 14 N.
(C) 25 N.
(D) 39 N.

Correct answer: (A)

Since the book is dragged at constant speed (along a constant direction), then Newton's first law applies, which means that the forces acting on the book must sum to zero, according to the Newton's law flowcharts.

Response (B) is the kinetic friction force f_k = mu_k*m*g; response (D) is the magnitude of either the weight or the normal force (w = mg or N). Response (B) is the difference between the magnitudes of weight (or normal force) and the kinetic friction force (m*g - mu_k*m*g), which are respectively along vertical and horizontal directions.

Student responses
Sections 4987, 4988
(A) : 16 students
(B) : 18 students
(C) : 2 students
(D) : 7 students