Physics quiz question: comparing directions and magnitudes of forces

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

"A 2019 Chevrolet Silverado LT Trail Boss, New York International Auto Show"
Kevauto
commons.wikimedia.org

A cargo load (of unknown mass) rests on the bed of a 10,660 kg stationary truck[*]. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Normal force of cargo load on the truck.
Normal force of ground on the truck.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)

[*] chevrolet.com/commercial/silverado-chassis-cab.

Correct answer (highlight to unhide): (D)

The truck has three forces acting on it:

Weight force of Earth on the truck (m_truck·g = 1.04×10⁵ N, downwards).
Normal force of cargo load on the truck (downwards).
Normal force of ground on the truck (upwards).

Because the truck is stationary, the magnitudes of the two downward forces added together must equal the magnitude of the upwards force, due to Newton's first law. So the normal force of cargo load on the truck and the normal force of ground on the truck, while being opposite in direction, cannot be equal in magnitude (the difference in magnitudes being equal to the weight of the truck).

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 22 students
(B) : 1 student
(C) : 9 students
(D) : 22 students

Success level: 41%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.65