AS Daily A Level Physics question
Which statement must be true when a small wooden block that initially floats at rest in water is covered by a thick layer of oil (less dense than water) so that, after settling, the block is completely submerged but still floating partly in water and partly in oil?
Answer
The correct answer is C.
Correct: C — The upthrust on the block is the same, and the volume of water displaced is smaller than before. A The upthrust on a floating object equals its weight, which is unchanged, and some buoyancy now comes from oil, so the water-displaced volume does not stay the same. B If it still floats, the upthrust cannot be smaller than the weight, and because some displaced fluid is lower-density oil, the total displaced volume must be larger, not smaller. C This follows from floating equilibrium (upthrust = weight) and the contribution of less-dense oil to buoyancy, so less water needs to be displaced. D With part of the displacement in less-dense oil, a larger total volume must be displaced to produce the same upthrust, not a smaller one.