Force balance at terminal velocity including buoyancy

Steps:

• At constant terminal speed, net force is zero: weight = viscous drag + thrust (upthrust/buoyancy).
• Weight acts downward; viscous drag and thrust act upward, opposing motion.
• For a steel ball in air, thrust (buoyant force) is negligible: thrust ≈ weight × (air density / steel density) ≈ 0.00015 × weight.
• Thus, viscous drag ≈ weight, so thrust < viscous drag < weight.

Why A is correct:

• Weight balances the sum of thrust and drag; buoyant thrust is tiny in air, making drag nearly equal to weight.

Why the others are wrong:

• B: Places viscous drag smallest, but drag dominates to balance weight.
• C: Places weight between drag and thrust, but weight is always largest.
• D: Places weight smallest, contradicting force balance where weight equals sum of others.

Final answer: A