Question

A ring of radius 1 m is lying on the surface of liquid. It is lifted from the liquid surface by a force of 4 newtons in such a way that the liquid film in it remains intact. The surface tension of liquid will be

• A. $\frac{1}{2\pi }N/m$
• B. $\frac{1}{\pi }N/m$
• C. $\frac{1}{3\pi }N/m$
• D. $\frac{1}{4\pi }N/m$

Answer 1

The correct option is B $\frac{1}{\pi }N/m$

Contect area $=2\times \left[Areaofring\right]$ because, water will pull it from both sides.

$T=\frac{F}{L}T=\frac{4}{2\times 2\pi r}=\frac{4}{4\pi r}=\frac{1}{\pi }N/m$