An air bubble of diameter $6\,mm$ rises steadily through a solution of density $1750\,kg / m ^3$ at the rate of $0.35\,cm / s$. The co-efficient of viscosity of the solution (neglect density of air) is $..........\,Pas$ (given, $g =10\,ms ^{-2}$)
A$5$
B$10$
C$8$
D$9$
JEE MAIN 2023, Medium
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B$10$
b Since the bubble is moving at constant speed the force acting on it is zero.
$B = F _{ V }$
$\frac{4}{3} \pi R ^3 \rho g =6 \pi \eta Rv$
$\eta=\frac{2 R ^2 \rho g }{9 v }=\frac{2 \times\left(3 \times 10^{-3}\right)^2 \times 1750 \times 10}{9 \times 0.35 \times 10^{-2}}=10\,Pas$
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