Physics quiz question: buoyant force on inflating pufferfish

Physics 205A Quiz 5, fall semester 2016
Cuesta College, San Luis Obispo, CA

"The Puffer Fish"

A long-spined pufferfish (Diodon holocanthus) swimming underwater expands its volume three-fold by gulping water into its stomach.[*] As a result, the magnitude of the buoyant force on the pufferfish:
(A) decreases.
(B) remains constant.
(C) increases.
(D) (Not enough information is given.)

[*] divingintaganga.blogspot.com/2010/09/how-and-why-pufferfish-inflate.html .

Correct answer (highlight to unhide): (C)

The magnitude of the buoyant force is given by:

FB = ρfluid·g·(Volume submerged),

where the density ρ is of the surrounding fluid (seawater) and the gravitational constant g = 9.80 m/s2. The volume of sea water displaced by the pufferfish increases, and thus the upwards buoyant force on it while increase in magnitude while it is filling with water. (However, since the fish is filling with water, the downwards weight force of gravity on increases as well.)

Sections 70854, 70855, 73320
Exam code: quiz05b0oM
(A) : 3 students
(B) : 11 students
(C) : 40 students
(D) : 1 student

Success level: 75%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.24

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