AS Daily A Level Physics question
A small boat floating on a still pond carries a dense rock. The rock is lifted out and dropped over the side where it sinks to the bottom. Neglect splashes and assume the pond is large. Which statement must be true about the water level at the shoreline after the rock has settled?
Answer
The correct answer is A.
Correct: A — It falls slightly, because the rock now displaces less water than when it was in the boat. A While in the boat the water displaced equalled the rock’s weight; once sunk the rock displaces only its own volume, and for a dense rock this volume is smaller, so the overall displaced volume and water level decrease. B This treats the rock’s volume as controlling both situations, but in the boat displacement was set by weight, which for a dense rock requires more water displaced than the rock’s own volume. C Conserving total mass does not fix the displaced volume; the governing condition switches from weight-based displacement (in boat) to volume-based displacement (submerged). D The fluid level change from a fully submerged object does not depend on depth; depth does not alter the volume of water displaced.