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

Options

A. ` 3.5 xx 10 ^(-2) N//m `
B. ` 7.0 xx 10 ^(-2) N//m `
C. `14.0 xx 10 ^(-2) N//m `
D. `1.08 N//m `

Answer

B

Explanation

`theta=0^(@):mg+2sigmaa=0.044`
`theta=180^(@):mg-2sigmaa=0.03`
`therefore" "4sigmaa=0.014Rightarrow0.2sigma=0.014`
`sigma=0.07N//m`