Physics quiz question: bus pushing car

Physics 205A Quiz 3, fall semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 4.16

A bus pushes a car along a horizontal road, such that they both travel to the right at constant speed. The mass of the bus is greater than the mass of the car. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Normal force of the bus on the car.
Normal force of the car on the bus.

(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): (C)

Although both the bus and car are subject to Newton's first law (both moving to the right at constant speed), the force of the bus pushing on the car and the force of the car pushing on the bus 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 (bus and car)

Thus the force of the bus pushing on the car and the force of the car pushing on the bus are a third law pair, and are equal in magnitude and opposite in direction due to Newton's third law.

Sections 70854, 70855
Exam code: quiz03Fn37
(A) : 18 students
(B) : 5 students
(C) : 31 students
(D) : 0 students

Success level: 67%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.53