A thin circular ring of radius 5 cm is placed gently on the surface of water. If the surface tension of water is 0.07 N m^ -1 , the excess force required to lift the ring from the surface is (Take π = (22)/(7) )

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Correct answer: C. 44 mN

For a thin circular ring, both the inner and outer boundaries are in contact with the liquid surface.

Total effective length of contact,

L = 2\pi R + 2\pi R = 4\pi R

Excess force required,

F = T \times L = T \times 4\pi R

F = 0.07 \times 4 \times \dfrac{22}{7} \times 0.05

F = 0.01 \times 4 \times 22 \times 0.05 = 0.044\text{ N}

F = 44\text{ mN}

Answer:

44\text{ mN}

A thin circular ring of radius 5 cm is placed gently on the surface...

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