A ring is cut from a platinum tube `8.5 cm` internal and `8.7 cm` external diameter. It is supported horizontally from the pan of a balance, so that it comes in contact with the water in a glass vessel. If an extra `3.103 gf` is required to pull it away from water, the surface tension of water is

Options

A. `72dyn//cm`
B. `70.80dyn//cm`
C. `63.35 dyn//cm`
D. `60 dyn//cm`

Answer

A

Explanation

`[2pixx8.7/2+2pi+8.5/2]sigma=3.97xx980`
or `sigma=(3.97xx980xx7)/(22xx17.2) dyn//cm^(1)=72dyn//cm^(1)`