**Stokes' law** gives the viscous drag on a small sphere of radius $r$ moving with velocity $v$ through a fluid of viscosity $\eta$ as:
A$F_v = 6\pi \eta r v$
B$F_v = \pi \eta r v$
C$F_v = 6\pi \eta r^2 v$
D$F_v = \eta r v / 6\pi$
Answer & Solution
Correct answer: A. $F_v = 6\pi \eta r v$
$F_v = 6\pi \eta r v$. Valid for laminar flow at low Reynolds number. Used to derive terminal velocity of a sphere falling in a fluid.
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