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Stokes' law: viscous drag on sphere of radius r moving at v through fluid of viscosity η:
AF = 4π η r
BF = η r²
CF = 6π η r v
DF = π v
Answer & Solution
Correct answer: C. F = 6π η r v
F_drag = 6π η r v. Valid for laminar (low Reynolds number). Used to derive terminal velocity of falling sphere; also Millikan's oil-drop experiment.
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