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Spherical balls of radius R fall through a viscous fluid of viscosity eta with velocity v. The retarding viscous (drag) force experienced by such a ball is:
- directly proportional to R but inversely proportional to v
- directly proportional to both the radius R and the velocity v
- inversely proportional to both the radius R and the velocity v
- inversely proportional to R but directly proportional to v
Correct answer: directly proportional to both the radius R and the velocity v
Solution
By Stokes' law, the viscous drag on a small sphere moving slowly through a fluid is F = 6*pi*eta*R*v. Hence F is directly proportional to both R and v.
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