A thin flat circular disc of radius $7.0 \mathrm{~cm}$ is placed gently over the surface of water. I

Question

A thin flat circular disc of radius $7.0 \mathrm{~cm}$ is placed gently over the surface of water. If the 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

Professor PC Thomas Classes · Answer

(b) : Surface tension of water, $T=0.07 \mathrm{~N} \mathrm{~m}^{-1}$. Radius of disc, $r=7.0 \mathrm{~cm}$. The excess force required to pull the disc away from the surface is due to the surface tension acting along the circumference of the disc. Therefore, $F = T 	imes 2\pi r$. Substituting the given values, $F = 0.07 	imes 2 	imes \frac{22}{7} 	imes 7 	imes 10^{-2} = 0.07 	imes 44 	imes 10^{-2} = 3.08 	imes 10^{-2} \mathrm{~N}$. Converting this to millinewtons gives $30.8 \mathrm{~mN}$.