Physics quiz question: specific acoustic impedance

Physics 205A Quiz 1, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Comprehensive Problem 1.77

The equation[*] for the specific acoustic impedance z of air is P/v, where P is the sound pressure, measured in units of kg·m^–1·s^–2, and v is the particle velocity, measured in units of m·s^–1. The units for the specific acoustic impedance z are:
(A) kg·m^–2·s^–1.
(B) kg·m^–2·s.
(C) kg^–1·m²·s^–1.
(D) kg^–1·m²·s.

[*] wki.pe/Acoustic_impedance.

Correct answer (highlight to unhide): (A)

Replacing the quantities in the equation with their kg, m and s units, except for the specific acoustic impedance z:

z = P×v^–1,

z = kg·m^–1·s^–2 × (m·s^–1)^–1,

z = kg·m^–1·s^–2 × m^–1·s,

z = kg·m^–2·s^–1.

Sections 70854, 70855
Exam code: quiz01bL0t
(A) : 48 students
(B) : 7 students
(C) : 4 students
(D) : 2 students

Success level: 79%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.60

Physics quiz question: sorptivity units

Physics 205A Quiz 1, fall semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 1.39

The equation for the volume V (measured in units of m³) of liquid absorbed over a time t (measured in units of s) by a dry porous medium is V = A⋅S⋅sqrt(t). The cross-sectional area A of the medium in contact with the liquid is measured in units of m². The units for the sorptivity[*] of the medium, S are:
(A) m ⋅ s^–1/2.
(B) m^–1 ⋅ s^1/2.
(C) m^–2 ⋅ s^1/2.
(D) m² ⋅ s^1/2.

[*] wki.pe/Capillary_force.

Correct answer (highlight to unhide): (A)

Replacing the quantities in the equation with their m and s units, except for the sorptivity S:

V = A⋅S⋅sqrt(t),

(m³) = (m²)⋅S⋅(s)^1/2,

then solving for S in terms of these units:

m ⋅ s^–1/2 = S.

Sections 70854, 70855
Exam code: quiz01unI7
(A) : 24 students
(B) : 7 students
(C) : 13 students
(D) : 13 students

Success level: 33%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.69