Question

A thin square plate of side `5 cm` is suspended vertically a balance so that lower side just dips into water with side to surface. When the plate is clean `(theta=0^@)`, it appears to weigh `0,044 N`. But when the plate is greasy `(theta=180^@)` it appears to weigh `0.03 N`. The surface tension of water is
A. `3.5xx10^(-2)N//m`
B. `7.0xx10^(-2)N//m`
C. `14.0xx10^(-2)N//m`
D. `1.08N//m`

Answer 1

Correct Answer - B
Difference in apparent weights is due to differences in forces of surface tension. Due to `180^@` the force surface tension in one case is opposite to the force of surface tension in the other case.
`:. 2xxsigma_(2)xx10/10=0.004-0.03`
or `sigma_(2)=0.014/1xx5N//m=0.07N//m`