Force balance for constant velocity descent

Steps:

• Submarine moves downward at constant velocity, so acceleration is zero and net force is zero.
• Weight acts downward; upthrust and viscous drag act upward (drag opposes downward motion).
• Sum of upward forces equals downward force: upthrust + drag = weight.
• Rearrange to find drag: drag = weight - upthrust.

Why D is correct:

• By Newton's first law, in equilibrium the drag magnitude equals the difference between weight and upthrust magnitudes.

Why the others are wrong:

• A: drag + upthrust equals weight, not a direct relationship for any single force.
• B: weight + drag exceeds total upward force, violating equilibrium.
• C: weight + upthrust is unrelated to drag and too large for balance.

Final answer: D