Fluid MechanicsMedium
Question
A thin flat circular disc of radius 4.5 cm is placed gently over the surface of water. If surface tension of water is $0.07\text{ }N{\text{ }m}^{- 1}$, then the excess force required to take it away from the surface is
Options
A.1.98 mN
B.99 N
C.19.8 mN
D.198 N
Solution
Sol. [Key Concept: Viscous drag on a disc resting on a liquid surface]
Excess force $= T \times 2\pi R$
$${= \frac{7}{100} \times 2 \times 3.14 \times \frac{4.5}{100} }{= 197.82 \times 10^{- 4} }{= 19.8 \times 10^{- 3}\text{ }N }{= 19.8mN }$$
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