Buoyancy and motion in uniform fluid

Steps:

• Upthrust equals weight of displaced water; since sphere is fully immersed and water density is uniform, upthrust remains constant regardless of depth.
• Weight of sphere is constant and less than upthrust, so net upward force (resultant force) is upthrust minus weight.
• As sphere rises, it accelerates upward, gaining speed.
• Increasing speed causes drag force to increase (proportional to speed squared), reducing the net upward resultant force over time.

Why A is correct:

• Upthrust is constant due to Archimedes' principle (displaced volume and fluid density unchanged); resultant force decreases as drag rises with velocity.

Why the others are wrong:

• B: Resultant force decreases, not increases, due to growing drag.
• C: Upthrust does not decrease in uniform density water.
• D: Upthrust is constant, not decreasing.

Final answer: A