Physics clicker question: equal magnitude, opposite direction forces

Physics 5A, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 4.50

Students were asked the following clicker questions (Classroom Performance System, einstruction.com) near the beginning of their learning cycle:


[0.6 participation points.] Which Newton's law tells you that the force of the Earth pullling on the book is equal in magnitude and opposite in direction to the force of the table pushing on the book?
(A) Newton's first law.
(B) Newton's second law.
(C) Newton's third law.
(D) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 25 students
(B) : 4 students
(C) : 10 students
(D) : 1 student

Correct answer: (A)
The force of the Earth pullling on the book and the force of the table pushing on the book sum to zero, as Newton's first law applies to the book.

After discussion, a related follow-up question was asked, with the following results.

[0.6 participation points.] Which Newton's law tells you that the force of the book pushing on the table is equal in magnitude and opposite in direction to the force of the table pushing on the book?
(A) Newton's first law.
(B) Newton's second law.
(C) Newton's third law.
(D) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 25 students
(B) : 3 students
(C) : 11 students
(D) : 1 student

Correct answer: (C)

The force of the book pushing on the table and the force of the table pushing on the book 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 book pushing on the table and the force of the table pushing on the book are a third law pair, and are equal in magnitude and opposite in direction due to Newton's third law.

At this point in the learning cycle, students were already familiar with Newton's first law, but had trouble engaging their just-learned knowledge of Newton's third law in recognizing third law pairs.