Physics quiz question: truck-pushed crate

Physics 205A Quiz 3, fall semester 2018
Cuesta College, San Luis Obispo, CA

A truck applies a force of 1,700 N to the right on a 400 kg crate, such that the crate (with the truck) moves to the right across a road with an acceleration of 
0.50 m/s². The road is not frictionless. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Applied force of truck on the crate.
Kinetic friction force of road on the crate.

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

Correct answer (highlight to unhide): (D)

The crate has two vertical forces acting on it:

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

Because the crate has no vertical motion, these vertical two forces must be equal in magnitude and opposite in direction, due to Newton's first law, such that these two forces produce a vertical net force of zero.

The crate has two horizontal forces acting on it:

Applied force of truck on the crate (1,700 N to the right).
Kinetic friction force of road on the crate (to the left, against the direction of motion).

Because the crate is accelerating (to the right), then from Newton's second law (as its motion is changing, and not constant) these two horizontal forces do not cancel out, and must sum to a horizontal net force that points in the direction of acceleration. Thus the applied force of truck on the crate is opposite in direction to, but is greater in magnitude than the kinetic friction force of road on the crate.

Sections 70854, 70855
Exam code: quiz03pRH5
(A) : 9 students
(B) : 12 students
(C) : 4 students
(D) : 31 students

Success level: 55%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.60