Q 11-09-005NEETNEET 2024Top questionMedium
A thin flat circular disc of radius $4.5$ cm is placed gently over the surface of water. If surface tension of water is $0.07\ \text{N m}^{-1}$, then the excess force required to take it away from the surface is :
Answer: (C) $19.8$ mN
Concept. Surface tension acts along the rim: $F = S \times 2\pi r$.
Step 1. Substitute.
$$F = 0.07 \times 2 \times 3.14 \times 0.045 \approx 0.0198\ \text{N} = 19.8\ \text{mN}$$
Answer. $19.8$ mN (Option C).
Solution by Sreeraj P, M.Sc Physics