Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 4.26, 4.95
A book on a horizontal table (with friction) has a force of 9.0 N applied on it as it moves with constant speed to the right. The magnitude of the normal force of the table on the book is 25 N. Newton's __________ law tells you that the magnitude of the normal force of the book on the table is also 25 N.(A) first.
(B) second.
(C) third.
(D) (Not enough information is given.)
Correct answer: (C)
Although the book is subject to Newton's first law (moving to the right at constant speed), the normal force of the table on the book and the normal force of the book on the table satisfy all three components of the POFOSTITO mnemonic (cf. Light and Matter by Benjamin Crowley, lightandmatter.com):
POF: Pair of Opposite Forces
OST: Of Same Type (in this case, normal forces)
ITO: Involving Two Objects (table and book)
Thus the force of the table pushing (up) on the book and the force of the book pushing (down) on the table are a third law pair, and are equal in magnitude and opposite in direction due to Newton's third law.
Student responses
Section 72177
(A) : 6 students
(B) : 0 students
(C) : 9 students
(D) : 0 students
Success level: 60%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.27
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