A uniform wire of 20 cm long is bent into a circle. It is placed gently on the surface of water of surface tension 0.07 Nm^-1. The extra force than its weight required to pull it out of the water is

Given,

$\mathrm{Surface} \mathrm{tension}= 0.07{\mathrm{Nm}}^{-1}$

We know that,

$\mathrm{T}=\frac{\mathrm{F}}{\mathrm{L}}$

$\mathrm{F}=\mathrm{TL}$

$\mathrm{L}=2\mathrm{\pi }r$

Because it is a ring with two circumferences that are in touch with the liquid and a circumference that is equal to the length of the wire, surface tension force works along the circumference.

$\mathrm{F}=\left(2\mathrm{T}\right)\mathrm{L}$

$\mathrm{F}=\left(2\mathrm{T}\right)\left(2\mathrm{\pi r}\right)$$; \mathrm{F}=2\times 0.07\times 20\times {10}^{-2}$

$\mathrm{F}=28\times {10}^{-3}\mathrm{N}$

$\mathrm{F}=0.028\mathrm{N}$