Physics quiz question: rotational inertia of Solowheel wheel

Physics 205A Quiz 5, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 8.2

"Riding the Solowheel to work - every days obstacle course"
SoloWheela
youtu.be/w2PATMZFEJo

A Solowheel motorized unicycle[*] rolls without slipping on a horizontal road with a wheel that can be approximated as a solid disk of radius 0.21 m and mass 5.0 kg. The rotational inertia of the Solowheel wheel is:
(A) 0.011 kg·m².
(B) 0.11 kg·m².
(C) 0.22 kg·m².
(D) 2.2 kg·m².

(Given: I_disk = (1/2)·M·R².)

[*] solowheel.com/solowheel-specs/.

Correct answer (highlight to unhide): (B)

Since the Solowheel wheel can be approximated as a solid disk,

I = (1/2)·m·r² = (1/2)·(5.0 kg)·(0.21 m)² = 0.11025 kg·m²,

or to two significant figures, the rotational inertia is 0.11 kg·m².

(Response (A) is (5.0 kg)·(0.21 m)/(100 kg); response (C) is the rotational inertia (5.0 kg)·(0.21 m)² for a ring (instead of a solid disk); response (D) is (1/2)·(100 kg)·(0.21 m)².)

Sections 70854, 70855, 73320
Exam code: quiz05LuF7
(A) : 6 students
(B) : 32 students
(C) : 10 students
(D) : 13 students

Success level: 52%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.39