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An iron block (density greater than water) is suspended from a fixed rigid support above by a massless string and is submerged in water inside a vessel. A wooden block (density less than water) is tied to the bottom of the same vessel by a massless string and floats submerged. The vessel now begins to accelerate horizontally to the right. Which of the following statements correctly describes the resulting deflection of the blocks?
- The iron block deflects toward the right.
- The wooden block deflects toward the right.
- The iron block deflects toward the left.
- The wooden block deflects toward the left.
Correct answer: The wooden block deflects toward the right.
Solution
When the vessel accelerates to the right, in the non-inertial frame a pseudo-force acts leftward on everything. The effective gravity tilts left-downward. The buoyant force on each block is directed opposite to effective gravity (i.e., right-upward). For iron (denser than water), weight > buoyancy in horizontal sense, so it deflects left. For wood (less dense), buoyancy > weight in horizontal sense, so it deflects right. Correct answers: B (wooden block right) and C (iron block left).
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