Physics quiz question: book on crate, on floor

Physics 205A Quiz 3, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 4.37, 4.53

A book and a crate are stacked on the floor, and both are motionless. The mass of the crate is greater than the mass of the book. Newton's __________ law tells you that these two forces are equal and opposite in direction:

Force of the crate pushing up 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): (A)

The normal force of the crate on the book is upwards, and the weight force of Earth on the book is downwards. These are the only two vertical forces on the book, and since the book's motion is constant (stationary), then from Newton's first law these forces must sum to zero, and thus be equal in magnitude and opposite in direction.

The normal force of the crate on the book and the weight force of the book do not 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 (normal force and weight force)
ITO: Involving Two Objects (crate, book, and Earth)

Thus the normal force of the crate on the book and the weight force of the book are not a third law pair (while being equal in magnitude and opposite in direction due to Newton's first law).

Sections 70854, 70855
Exam code: quiz03sQr7
(A) : 31 students
(B) : 2 students
(C) : 21 students
(D) : 1 student
(E) : 0 students

Success level: 56%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64