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Question
A raindrop of radius 0.3 mm falls through the air with a terminal velocity of 1 m/s. The viscosity of air is 18 x 10−6 Ns /m2. Find the viscous force on the raindrop.
Solution
Given:
r = 0.3 mm = 3 × 10−4 m,
v = 1 m/s,
η = 18 × 10−6 Ns/m2
To find:
Viscous force (F)
Formula:
F = 6πηrv
Calculation:
From formula,
F = 6 × 3.142 × 18 × 10−6 × 3 × 10−4 × 1
= 324 × 3.142 × 10−10
= antilog {log 324 + log 3.142} × 10−10
= antilog {2.5105 + 0.4972} × 10−10
= antilog {3.0077} × 10−10
= 1.017 × 103 × 10−10
= 1.017 × 10−7 N
The viscous force is 1.017 × 10−7 N.
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