A thin flat circular disc of radius 4.5 cm is placed gently over the surface of water. If surface te

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 $0.07 \mathrm{Nm}^{-1}$, then the excess force required to take it away from the surface is

Professor PC Thomas Classes · Answer

(a) : Surface tension of water, $T=0.07 \mathrm{~N} \mathrm{~m}^{-1}$ Radius of disc, $r=4.5 \mathrm{~cm}$ \ Required excess pressure, $P=\frac{2 T}{r}$ \ ∴ $\quad$ Required force to take it away from the surface is \ $F=(P) 	imes$ Area or $\quad F=\frac{2 T}{r} 	imes \pi r^2=2 	imes T 	imes(\pi r)$ $=2 	imes 0.07 	imes 3.14 	imes 4.5 	imes 10^{-2}=1.978 	imes 10^{-2}=19.8 \mathrm{mN}$