Physics quiz question: dynamic pressure

Physics 205A Quiz 1, fall semester 2019
Cuesta College, San Luis Obispo, CA

The drag force F_drag (measured in units of kg·m·s^–2) experienced by an object moving through a fluid is given by[*]:

F_drag = q·A_s·C_D.

The surface area A_s is measured in units of m², and the drag coefficient C_D is a dimensionless quantity. The units for the dynamic pressure q are:
(A) kg^–1·m·s².
(B) kg^–1·m³·s².
(C) kg·m^–1·s^–2.
(D) kg·m^–3·s^–2.

[*] wki.pe/Parasitic_drag#Description.

Correct answer (highlight to unhide): (C)

Starting with the expression for the drag force F_drag:

F_drag = q·A_s·C_D,

the dynamic pressure q can be expressed as:

q = F_drag/(A_s·C_D).

Then replacing the quantities in the equation with their kg, m and s units, the units of q are:

q = kg·m·s^–2/((m²)·(1)),

q = kg·m·s^–2·m^–2,

q = kg·m^–1s^–2.

Sections 70854, 70855
Exam code: quiz01B34t
(A) : 2 students
(B) : 2 students
(C) : 49 students
(D) : 2 students

Success level: 89%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.25