Question

A $10cm$ long wire is placed horizontally on the surface of water and is gently pulled up with a force of $2\times {10}^{-2}N$. To keep the wire in equilibrium, the surface tension of water in ${Nm}^{-1}$ is.

• A. $0.1$
• B. $0.2$
• C. $0.001$
• D. $0.002$

Answer 1

The correct option is A $0.1$

As the liquid film has 2 surfaces hence,
$F=S\times 2l$
$\Rightarrow S=\frac{F}{2l}=\frac{2\times {10}^{-2}N}{2\times (10\times {10}^{-2}m)}$
$S={10}^{-1}N/m=0.1N/m$