Question

A thin film of water of thickness 80 mm is sandwiched between the glass plates and forms a circular patch of radius 12 cm. The normal force required to separate the plates is (surface tension of water is 0.072 N/m)

• A. 8.14 N
• B. 81.4 N
• C. 40.7 N
• D. 0.0814 N

Answer 1

The correct option is D 0.0814 N

The normal force required will be $F=\frac{2\sigma A}{d}$
where, $\sigma$ = Surface Tension , d = thickness , A = area of circular path
$F=\frac{2\times 0.072\times \left(\pi {0.12}^2\right)}{0.08}=0.0814N$