Question

Calculate the force required to separate the glass plate of area ${10}^{-2}{m}^2$ with a film of water $0.05mm$ thick [surface tension of water is $70\times {10}^{-3}N/m$]:-

• A. $25N$
• B. $20N$
• C. $14N$
• D. $28N$

Answer 1

The correct option is D $28N$

from the definition of surface tension $s.t=\frac{force}{length}$---(1)

it is a glass plate to we have to include the are of both the areas,hence $2\times area=length\times thickness$

$length=\frac{2\times area}{thickness}=\frac{2\times {10}^{-2}\times 1000}{0.05}m=400m$

so,put in equation (1), we will find

$force=s.t\times length=70\times {10}^{-3}\times 400=28N$