Topic: Surface energy, surface tension, angle of contact, excess pressure, drops, bubbles, capillary rise· Properties of Solids and Liquids

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 :

1. 99 N

2. 19.8 mN

3. 198 N

4. 1.98 mN

Doubtly Asked 30 Oct 2026 NEET UG 2024 Practice test neet Save WhatsApp Report

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Answer: 2. 19.8 mN

Key idea: The surface-tension force acts along the line of contact between the disc and the water, which is the circumference of the disc, \(2\pi r\).

Excess force \(F=T\times 2\pi r\).

\(F=0.07\times2\times3.14\times0.045\approx1.98\times10^{-2}\) N \(=19.8\) mN.

Option 2. (Counting both faces of the disc would double this to 39.6 mN, which is not an option, so the single contact line is intended.)

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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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 :

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