Physics quiz question: net force on accelerating sliding suitcase

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA

A force of 85 N is applied to the right on a 25 kg suitcase that is already sliding to the right, such that it accelerates to the right with a magnitude of 1.5 m/s². The horizontal floor is not frictionless. The magnitude of the net force on the suitcase is:
(A) 0 N.
(B) 38 N.
(C) 85 N.
(D) 2.5×10² N.

Correct answer (highlight to unhide): (B)

Since the suitcase is accelerating to the right, Newton's second law applies to its horizontal motion, such that the net force must be non-zero. There are two horizontal forces acting on the suitcase:

The applied force on suitcase (to the right, magnitude N = 85 N).
Kinetic friction force of the floor on suitcase (to the left, opposing its motion, magnitude f_k = ?).

Since the suitcase is accelerating to the right, the net force points to the right, and has a magnitude of

ΣF_x = m·a_x,

ΣF_x = (25 kg)·(9.80 N/kg) = 37.5 N,

or to two significant figures, the net force has a magnitude of 38 N.

(Response (A) only applies if the suitcase were stationary, or sliding horizontally at a constant speed, such that Newton's first law would apply; response (C) is the magnitude of the applied force (which would be the magnitude of the net force if horizontal floor were frictionless (but that would mean that the magnitude of the acceleration to the right would be different than the given value of 1.5 m/s²); response (D) is the magnitude of the weight force w = m·g.)

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 4 students
(B) : 24 students
(C) : 24 students
(D) : 5 students

Success level: 42%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66