Question

A drop of water placed between two parallel glass plates which are then pressed together until the water film is 0.01 cm thick. If the area of water in contact with either plate is 5 cm², then the force due to surface tension drawing the plates together is ______.

(S.T. of water is 75 dyne/cm)

Options

• 75 gm-wt

• 150 gm-wt

• 750 gm-wt

• 425 gm-wt

Answer 1

A drop of water placed between two parallel glass plates which are then pressed together until the water film is 0.01 cm thick. If the area of water in contact with either plate is 5 cm², then the force due to surface tension drawing the plates together is 75 gm-wt.

Explanation:

`Delta "P" = "T" (1/"r"_1 + 1/"r"_2)`

As r₁ = r and r₂ = `alpha`,

`Delta "P" = "T"/"r"` But r = d/2

∴ `Delta "P" = "2T"/"d"`

∴ F = P.A = `"2T"/"d" "A" = (2 xx 75 xx 5)/0.01`

= 75 x 10³ dyne

= 75 gm-wt