A thin flat circular disc of radius 4.5 cm is placed gently over the surface of water. If surface tension of water is 0.07 Nm^–1, then the excess force required to take it away from the surface is :

The correct answer is 19.8 mN.

Given:

Radius of the circular disc  r = 4.5 cm = 0.045 m

Surface tension of water  γ = 0.07 N·m⁻¹

When the disc is gently placed on the water, the surface tension acts along the circular edge (the contact line) and provides an upward force equal to the surface‑tension coefficient multiplied by the length of that edge.

Length of the contact line (the circumference):

Excess force due to surface tension:

Convert newtons to millinewtons (1 N = 1000 mN):

Thus the excess force required to lift the disc away from the water surface is 19.8 mN.