Physics quiz question: truck pushing bale, accelerating

Physics 205A Quiz 3, fall semester 2015
Cuesta College, San Luis Obispo, CA

A truck pushes on a bale of hay, such that they both accelerate together to the right. The mass of the truck is less than the mass of the bale. The magnitude of the normal force of the truck on the bale is __________ the magnitude of the normal force of the bale on the truck.
(A) equal to.
(B) less than.
(C) greater than.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

Although both the truck and bale are subject to Newton's second law (both accelerating to the right), the normal force of the truck pushing on the bale and the normal force of the bale pushing on the truck 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 (truck and bale)

Thus the normal force of the truck pushing on the bale and the normal force of the bale pushing on the truck are a third law pair, and are equal in magnitude and opposite in direction due to Newton's third law. (This is true regardless if the truck and bale are both stationary, moving together at constant speed, or accelerating together.)

Sections 70854, 70855, 73320
Exam code: quiz03re3T
(A) : 29 students
(B) : 7 students
(C) : 54 students
(D) : 0 students

Success level: 40%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.40