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