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

• A

  198 N
• B

  1.98 mN
• C

  99 N
• Correct answer
  D

  19.8 mN

Solution

$\begin{aligned} & \text { Excess force }=T \times 2 \pi R \\ & =\frac{7}{100} \times 2 \times 3.14 \times \frac{4.5}{100} \\ & =197.82 \times 10^{-4} \\ & =19.8 \times 10^{-3} \mathrm{~N} \\ & =19.8 \mathrm{mN}\end{aligned}$

Hence, the correct answer is option: 'D'.