Physics quiz question: net force on stacked coffee filters

Physics 205A Quiz 3, fall semester 2015
Cuesta College, San Luis Obispo, CA

A stack of paper basket coffee filters increases in downwards speed for a brief amount of time just after it is released. Air resistance is not negligible. During this brief amount of time, the direction of the net force exerted on the stacked filters is:
(A) upwards.
(B) downwards.
(C) (Has no direction, as the magnitude is zero.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

The increasing speed of the stacked filters means that Newton's second law applies to the motion of the stacked filters, such that the net force must be non-zero. Since the stacked filters is moving downwards as its speed is increasing, its acceleration must be in the same direction as its motion, and thus must point downwards. From Newton's second law, the net force and acceleration must have the same direction:

ΣF_y = m·a_y,

and since the acceleration points downwards, then the net force on the stacked filters must also point downwards.

Sections 70854, 70855, 73320
Exam code: quiz03re3T
(A) : 10 students
(B) : 60 students
(C) : 4 students
(D) : 0 students

Success level: 81%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52