Question

The rings of radius 1m is lying on the surface of liquid. It is lifted from the liquid surface by a force of 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

Answer: (B)

$\frac{1}{\pi }N/m$

The force acting on the ring against the lift=$2\times Tl$

where $l$ is the length of ring in contact with water surface, and factor of $2$ is included because of the inner and outer surfaces of the ring

$⟹4=2\times T\times \left(2\pi r\right)$

$⟹T=\frac{1}{\pi }N/m$