Question

Calculate the viscous force acting on a rain drop of diameter 1 mm, falling with a uniform velocity 2 m/s through air. The coefficient of viscosity of air is 1.8 × 10^-5 Ns/m².

Answer 1

Data : d = 1 mm, v₀ = 2 m / s, η = 1.8 × 10^-5 N.s/m²

r = d/2 = 0.5 mm = 5 × 10 m^-4 m

By Stokes’ law, the viscous force on the raindrop is f = 6πηrv₀

= 6 × 3.142 (1.8 × 10^-5 N.s/m² × 5 × 10^-4 m)(2 m/s) 

= 3.394 × 10^-7 N