Physics quiz question: kinetic friction force on sliding suitcase

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. If the magnitude of the applied force is slightly increased to 11 N such that the suitcase will slow down at a faster rate while still moving to the right, the magnitude of the kinetic friction force on the suitcase will:
(A) increase.
(B) remain the same.
(C) decrease.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

The kinetic friction force of the floor on suitcase points to the left, against the direction of motion, and has a magnitude of:

fk = μk·N,

where μk is the kinetic friction coefficient, a property of the suitcase and the floor, and is a constant; while N is the magnitude of the normal force of the floor on the suitcase, which by Newton's first law is equal to the magnitude of the weight force of Earth on the suitcase, also a constant. Thus increasing the applied force on the suitcase will not change the magnitude of the kinetic friction force on the suitcase, as long as it has already been "unstuck" and is moving.

Sections 70854, 70855, 73320
Exam code: quiz03fkfs
(A) : 20 students
(B) : 36 students
(C) : 16 students
(D) : 0 students

Success level: 50%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.57

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