AQA AS Level Physics Paper 2, November 2021: Question 28

✅ Correct Answer: C

The terminal speed of Y is greater than that of Z.

At terminal speed, the net force acting on the falling object is zero. Therefore, the downward force (weight minus upthrust) equals the upward viscous drag force. Comparing the properties and forces of objects Y and Z leads directly to option C being the only physically consistent statement among the choices.

💡 Key Knowledge

• Condition for Terminal Speed: Weight ($W$) = Upthrust ($U$) + Viscous Drag ($F$).
• Stokes' Law Context: For small spherical objects in laminar flow, terminal speed v = 2r²(ρ - σ)g / 9η , where r is radius, ρ is object density, σ is fluid density, and η is viscosity.
• Larger masses, greater densities, or smaller cross-sectional areas typically lead to higher terminal speeds.

🧠 Exam Technique

• In multiple-choice questions involving inequalities, test each option systematically by recalling the governing equations.
• Eliminate blatantly incorrect statements first by checking extreme cases (e.g., density differences or size scaling).
• Trust your derivation of the terminal velocity formula rather than guessing based on letter pairings.

❌ Common Errors

• Forgetting to account for upthrust, considering only weight and drag.
• Confusing the dependence on radius—terminal speed scales with the square of the radius ( r² ), meaning small changes in size have a massive impact.
• Assuming all objects reach the same terminal speed regardless of mass or surface area.