The terminal velocity of a copper ball of radius $5\,mm$ falling through a tank of oil at room temperature is $10\,cm\,s ^{-1}$. If the viscosity of oil at room temperature is $0.9\,kg\,m ^{-1} s ^{-1}$, the viscous drag force is :
NEET 2022, Easy
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$F\,=6 \pi \eta rv$
$\quad =6 \times 3.14 \times 0.9 \times 5 \times 10^{-3} \times 10 \times 10^{-2}$
$\quad=847.8 \times 10^{-5}\,N$
$\quad=8.48 \times 10^{-3}\,N$
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