A2 Daily A Level Physics question
A cylindrical fishing float stands upright in fresh water with 75% of its volume submerged. It is then placed in a liquid that is 1.2 times as dense as fresh water. Assuming the float does not absorb liquid and its mass is unchanged, what fraction of its volume will be submerged in the denser liquid?
Answer
The correct answer is B.
Correct: B — 0.625. In fresh water the float’s effective density is 0.75 of water; in a fluid 1.2 times denser, the submerged fraction is 0.75/1.2 = 0.625 by balancing weight with upthrust per unit volume. A 0.75 assumes the submerged fraction is unchanged when the fluid changes, ignoring that greater fluid density increases upthrust per unit submerged volume. B 0.625 is correct because the fraction submerged equals (object density)/(fluid density), so 0.75/1.2 = 0.625. C 0.90 reverses the effect, as a denser fluid provides more upthrust for the same volume, so the object should sit higher, not deeper. D 0.60 treats the 20% increase in fluid density as a simple 20% decrease from 0.75, but the correct relation is proportional (divide by 1.2), not subtract 0.2.