Physics midterm question: truck pushing bale, constant speed

Physics 205A Midterm 1, fall semester 2011
Cuesta College, San Luis Obispo, CA

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

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

Normal force of the truck on the bale.
Friction force of the road on the bale.

(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 bale has two vertical forces acting on it:

Weight force of Earth on bale (downwards).
Normal force of road on bale (upwards).

Because the bale is stationary in the vertical direction, these two forces are equal in magnitude and opposite in direction, due to Newton's first law along the vertical direction.

The bale has two horizontal forces acting on it (as it moves to the right):

Normal force of the truck on bale (to the right).
(Kinetic) friction force of road on bale (to the left).

Since the bale (along with the truck) is moving to the right at constant speed, then these two forces are equal in magnitude and opposite in direction, due to Newton's first law along the horizontal direction.

Sections 70854, 70855
Exam code: midterm01w4Sh
(A) : 16 students
(B) : 8 students
(C) : 19 students
(D) : 10 students

Success level: 30%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64