Terminal Velocity Force Balance

Steps:

• At terminal velocity, net force on bubble is zero for constant speed.
• Buoyant force (upward) balances weight (downward) plus viscous drag (downward, opposing rise).
• Diagram shows X upward (buoyancy), Y and Z downward.
• Balance gives X = Y + Z; Z is viscous drag, Y is bubble weight.

Why A is correct:

• Newton's first law requires buoyant force X to equal sum of weight Y and viscous force Z at equilibrium.

Why the others are wrong:

• B: Misidentifies Z as weight and Y as viscous, contradicting diagram labels.
• C: Correct equation but misidentifies forces, ignoring diagram's force directions.
• D: Implies net upward force, causing acceleration, not constant velocity.

Final answer: A