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 N \(m^{–1}\) , then the excess force required to take it away from the surface is 

(1) 19.8 mN 

(2) 198 N 

(3) 1.98 mN 

(4) 99 N

Answer 1

Correct option is : (1) 19.8 mN 

Excess force = \(T \times2\pi R\) 

\(=\frac{7}{100}\times2\times3.14\times \frac{4.5}{100}\)

\(= 197.82\times10^{-4}\)

\(= 19.8\times10^{-3}\ N\)

\(= 19.8\ mN\)