20190826

Physics quiz question: dynamic pressure

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

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

Fdrag = q·As·CD.

The surface area As is measured in units of m2, and the drag coefficient CD 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 Fdrag:

Fdrag = q·As·CD,

the dynamic pressure q can be expressed as:

q = Fdrag/(As·CD).

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

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