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AS Daily A Level Physics question

2026-06-08 OCR A Archimedes’ principle (M3.2) 3.2 Forces in action — pressure in fluids; upthrust and Archimedes’ principle (OCR Physics A AS)

A wooden block floats in fresh water. A 1 kg sealed metal tool can either be placed gently on top of the block (remaining above the water) or suspended from the block by a light string so the tool is fully submerged but not touching the bottom. In both cases the system is at rest. Which statement must be true when the tool is submerged compared with when it is on top?

  1. A The upthrust on the block is greater, because the total system displaces more water.
  2. B The block is less submerged, because some of the tool’s weight is supported directly by the water. (correct)
  3. C The depth the block sinks is unchanged, because the attached mass is the same in both cases.
  4. D The block is more submerged, because the submerged tool increases the pressure acting on the block.

Answer

The correct answer is B.

Correct: B — The block is less submerged, because some of the tool’s weight is supported directly by the water. A is wrong because the upthrust on the block equals the weight it must support; with the tool submerged, water provides upthrust on the tool so the block requires less upthrust, not more. B is correct because the water’s buoyant force on the tool reduces the string tension on the block, so it displaces less water. C is wrong because the immersion depends on supported weight, and the effective extra load is the tool’s weight minus its upthrust. D is wrong because hydrostatic pressure at the block is set by water depth; the tool below does not increase pressure on the block and only a reduced tension is transmitted.

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