A dust particle of radius 1.0 × 10⁻⁵ m moves at 0.50 m/s through air of viscosity 1.8 × 10⁻⁵ Pa s. What is the Stokes drag force on it?
Correct answer C. 1.7 × 10⁻⁹ N
Explanation
F = 6πηrv = 6π × 1.8 × 10⁻⁵ × 1.0 × 10⁻⁵ × 0.50 ≈ 1.7 × 10⁻⁹ N. Omitting π gives the 5.4 × 10⁻¹⁰ N distractor.
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