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A thin flat circular disc of radius 4.5 cm is placed gently over the surface of water. If surface tension of water is $0.07 \mathrm{~N} \mathrm{~m}^{-1}$, then the excess force required to take it away from the surface is
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19.8 mN
$\begin{aligned} & \text { Excess force }=T \times 2 \pi R \\ & =\frac{7}{100} \times 2 \times 3.14 \times \frac{4.5}{100} \\ & =197.82 \times 10^{-4} \\ & =19.8 \times 10^{-3} \mathrm{~N} \\ & =19.8 \mathrm{mN}\end{aligned}$
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