Net force zero at constant speed

Steps:

• Identify forces: gravitational (weight, downward), buoyant (upward), viscous drag (upward, opposes motion).
• At constant speed (terminal velocity), acceleration is zero, so net force equals zero by Newton's first law.
• Weight magnitude equals sum of buoyant and drag magnitudes.
• Correct diagram shows weight downward, with upward buoyant and drag vectors adding to equal weight.

Why B is correct:

• B depicts weight downward equaling the vector sum of upward buoyancy and drag, satisfying equilibrium condition (∑F = 0).

Why the others are wrong:

• A shows unbalanced forces, with upward forces not equaling weight.
• C incorrectly shows drag downward, opposing buoyancy.
• D has buoyant force larger than weight, implying upward acceleration.

Final answer: B