A Levels Physics (9702)•9702/12/O/N/19

Explanation
Buoyancy Opposes Downward Displacement
Steps:
- Cylinder floats partially, so buoyant force equals weight; pushing down increases submersion and buoyant force.
- Displacement of cylinder is downward during submersion process.
- Applied force F acts downward, parallel to displacement.
- Buoyant force (thrust) acts upward, antiparallel to displacement; water level rises as volume displaces.
Why D is correct:
- Work by thrust = ∫ thrust · ds; since thrust upward and ds downward, dot product negative, so thrust does negative work on cylinder (opposes motion per work-energy theorem).
Why the others are wrong:
- A: Displaced water rises, raising center of mass and increasing its gravitational potential energy.
- B: Cylinder moves downward, decreasing its gravitational potential energy.
- C: F downward parallel to downward ds, so positive work done by F on cylinder.
Final answer: D
Topic: Gravitational potential energy and kinetic energy
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