Question

A thin square plate of side $5cm$ is suspended vertically from a balance so that lower side just dips into water with side to surface. When the plate is clean $(\theta =0^{\circ })$, it appears to weigh $0.044N$. But when the plate is greasy $(\theta ={180}^{\circ })$, it appears to weigh $0.03N$. The surface tension of water

• A. $3.5\times {10}^{-2}N/m$
• B. $7.0\times {10}^{-2}N/m$
• C. $14.0\times {10}^{-2}N/m$
• D. $1.08N/m$

Answer 1

Answer: (B)

$7.0\times {10}^{-2}N/m$

Difference in apparent weights is due to differences in forces of surface tension. Due to ${180}^{\circ }$, the force surface tension in one case is opposite to the force of surface tension in the other case.
$\therefore 2\times {\sigma }_w\times \frac{10}{100}=0.004-0.03$
or ${\sigma }_w\times \frac{0.014}{1}\times 5N/m=0.07N/m$.