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 N m⁻¹, then the excess force required to take it away from the surface is (a) 19.8 mN (b) 198 N (c) 1.98 mN (d) 99 N

Answer: (a). Extra force = surface tension along the rim: F = T × 2π r. F = 0.07 × 2π × 0.045 = 0.0198 N. F = 19.8 mN. So the answer is (a).

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Force due to Surface Tension

A thin flat circular disc of radius $4.5\text{ cm}$ 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

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Answer: (a)

Extra force = surface tension along the rim: $F = T \times 2\pi r$ .
$F = 0.07 \times 2\pi \times 0.045 = 0.0198\text{ N}$ .
$F = 19.8\text{ mN}$ . So the answer is (a).

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