Physics quiz question: partially submerged block resting on bottom

Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA

A solid block has 75% of its volume below water, while resting on the bottom of a water tank. The force with the largest magnitude is the:
(A) weight force of Earth on the block.
(B) buoyant force of water on the block.
(C) normal force of tank bottom on the block.
(D) (There is a tie.)
(E) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

The block has three vertical forces acting on it:

Weight force of Earth on block (downwards, magnitude w = m·g).
Buoyant force of water on block (upwards, magnitude F_B = ρ_fluid·g·(Volume submerged)).
Normal force of tank bottom on block (upwards, magnitude N).

Because the block is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the two upwards forces (buoyant force and normal force) are together equal to the one downwards force (weight), such that the weight force has the largest magnitude of these three forces.

Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 30 students
(B) : 2 students
(C) : 5 students
(D) : 14 students
(E) : 0 students

Success level: 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67