Physics midterm question: forces on pushed-down book

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.37

A force of 1.0 N pushes down on a 0.50 kg book, which is stationary on a table. Newton's __________ law tells you that these two forces are equal and opposite in direction:

The applied force pushing on the book.
Weight force of the book.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)
(E) (Not enough information is given.)

Correct answer (highlight to unhide): (D)

The book has three vertical forces acting on it:

Weight force of Earth on book (downwards, magnitude w = m·g = 4.9 N).
Normal force of table on book (upwards, magnitude N = ?).
Applied force pushing on book (downwards, magnitude F_applied = 1.0 N).

Because the book is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the two downwards forces (weight and applied force) are together equal to the one upwards force (normal), such that the normal force must have a magnitude of 5.9 N. Thus the two forces listed above (applied force and weight force) are neither equal in magnitude (1.0 N and 4.9 N, respectively) nor opposite in direction (both are downwards).

Sections 70854, 70855, 73320
Exam code: midterm01p0To
(A) : 21 students
(B) : 3 students
(C) : 8 students
(D) : 41 students
(E) : 0 students

Success level: 56%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66