Physics quiz question: comparing forces on accelerated book

Physics 205A Quiz 3, fall semester 2009
Cuesta College, San Luis Obispo, CA

A book on a horizontal table (with friction) has a force of 9.0 N applied on it as it accelerates while moving to the right. The magnitude of the normal force of the table on the book is 25 N. Newton's __________ law allows you to determine that the magnitude of the kinetic friction force of the table on the book must be less than 9.0 N.
(A) first.
(B) second.
(C) third.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

The book has two vertical forces acting on it:

Weight force of Earth on book (downwards).
Normal force of table on book (25 N, upwards).

Because there is no vertical motion for the book as it slides across the horizontal table, these two forces must be equal in magnitude (each 25 N) and opposite in direction, due to Newton's first law.

The book has two horizontal forces acting on it:

Applied force on the book (9.0 N, to the right).
Kinetic friction force of the table on the book (to the left, opposite the direction of motion).

Since the book is accelerating as it moves to the right, by Newton's second law the net force must point to the right, such that the kinetic friction force of the table on the book (directed to the left) must have a magnitude less than the applied force (directed to the right) of 9.0 N.

Student responses
Sections 70854, 70855
(A) : 5 students
(B) : 37 students
(C) : 6 students
(D) : 0 students

Success level: 77%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.62