Physics quiz question: net force on sliding suitcase, slowing down

Physics 205A Quiz 3, fall semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.85

A force of 10 N is applied to the left on a suitcase that is already sliding to the right, such that it slows down. The floor is not frictionless. The direction of the net force on the suitcase is to the:
(A) left.
(B) right.
(C) (has no direction, as the net force is zero.)
(D) (Not enough information given.)

Correct answer (highlight to unhide): (A)

The suitcase has two vertical forces acting on it:

Weight force of Earth on suitcase (downwards).
Normal force of floor on suitcase (upwards).

Because the suitcase has no vertical motion, these two vertical forces are equal in magnitude and opposite in direction, due to Newton's first law.

The suitcase has two horizontal forces acting on it:

Kinetic friction force of floor on suitcase (to the left, against the direction of motion).
External applied force on suitcase (10 N, to the left).

Because the suitcase slows down as it moves to the right, the acceleration points to the left, which by Newton's second law is also the direction of the net force on the suitcase. (Also because the only two horizontal forces acting on the suitcase point to the left, then they add up to a net force that points to the left.)

Sections 70854, 70855, 73320
Exam code: quiz03fkfs
(A) : 43 students
(B) : 24 students
(C) : 5 students
(D) : 0 students

Success level: 60%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.74