Physics quiz question: work done against falling coffee filter stack

Physics 205A Quiz 4, fall semester 2017
Cuesta College, San Luis Obispo, CA

"Coffee Filter Terminal Velocity Slo-mo"
LeviNeuzerling
youtu.be/sBwK9IhEd64

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. Identify the force(s) that do negative work on the coffee filter stack just after it is released.
(A) w.
(B) F_drag.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

Correct answer (highlight to unhide): (B)

There are two forces acting on the coffee filter stack just after it is released:

weight force w of Earth on stack (downwards),
drag force F_drag of air on stack (upwards);

and as the coffee filter stack falls, its displacement (and direction of travel) points downwards.

The work done by the weight force is given by:

W_weight = (w·cos(0°))·s,

where the tail-to-tail angle between the direction of the weight force (downwards) and the displacement (downwards) is 0°. Since cos(0°) = +1, the weight force does positive work on the coffee filter.

The work done by the drag force is given by:

W_drag = (F_drag·cos(180°))·s,

where the tail-to-tail angle between the direction of the drag force (upwards) and the displacement (downwards) is 180°. Since cos(180°) = –1, the drag force does negative work on the coffee filter.

Sections 70854, 70855
Exam code: quiz04w33N
(A) : 22 students
(B) : 24 students
(C) : 1 student
(D) : 1 student

Success level: 50%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.89