MHT CET 2025 Physics question

A disc of paper of radius 'R' has a hole of radius 'r'. It is floating on a liquid of surface tension 'T'. The force of surface tension on the disc is

(A) T (R-r)
(B) T (R+r)Correct
(C) T-R
(D) T (R+r)

Answer: (B)

Solution

The force of surface tension is (length of the line of contact between the liquid surface and the body). The floating paper disc is in contact with the liquid surface along two circles: the outer edge of length and the edge of the hole of length . Hence $F = T(2π R+2π r)=2π

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Question ID VK-102475