Find the attractive force is newton between two parallel glass plates, separated by a distance `h=0.1mm` after a water drop of mass `m=70mg` was introduced between them. Assume wetting to be complete and surface tension of water, `T=70` dyne/cm

Explanation

The pressure inside the film is less than that outside by an amount `alpha((1)/(r_1) +(1)/(r_2))` where `r_1` and `r_2` are the principle radii of curvature of the meniscus. One of these is small being given by `h = 2 r_1 cos theta` while the other is large and will be ignored. Then `F ~~ (2 A cos theta)/(h) alpha` where `A =` area of the water film between the plates.
Now `A = (m)/(rho h)` so `F = (2 m alpha)/(rho h^2)` when `theta` (the angle of contact) `= 0`.