Physics quiz question: raising the sunken Mighty Servant 3

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

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 9.27, 9.31

"Mighty Servant 3 carrying her last cargo, the semi-submersible drilling rig GSF Aleutian Key"

In December 6, 2006 the ship Mighty Servant 3 accidentally sank, resting fully submerged on the ocean floor.[*] (No lives were lost in this incident.) The Mighty Servant 3 was later raised by filling its water-filled compartments with air. Up until the point where it just began to rise off of the ocean floor, its weight __________, while the buoyant force on it __________ in magnitude.
(A) decreased; decreased.
(B) decreased, remained constant.
(C) decreased; increased.
(D) increased; decreased.
(E) increased; remained constant.
(F) increased; increased.

[*] cargolaw.com/2006nightmare_mightyserve3.html.

Correct answer (highlight to unhide): (B)

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 ship, while still completely submerged, does not change, and thus the buoyant force on it still has the same magnitude while it is filling with air. However, since air is replacing the seawater in its compartments, the weight of the ship decreases.

Sections 70854, 70855, 73320
Exam code: quiz05LuF7
(A) : 5 students
(B) : 31 students
(C) : 23 students
(D) : 1 student
(E) : 1 student
(F) : 0 students

Success level: 51%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50

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