A wire is bent at an angle θ . A rod of mass m can slide along the bended wire without friction as shown in figure. A soap film is maintained in the frame kept in a vertical position and the rod is in equilibrium as shown in the figure. If rod is displaced slightly in vertical direction, then the time period of small oscillation of the rod is

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2πlg

2πlcos⁡θg

2πlgcos⁡θ

2πlgtan⁡θ

answer is A.

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Detailed Solution

Let S be the surface tension of the soap film. For equilibrium of rod2(l+y)tan⁡θ2mg=Fsurface 1mg=2ltan⁡θ2S×2; mg=4Sltan⁡θ2If the rod is displaced from its mean position by small displacement y, then restoring force on the rod is Frest =−Fsurface 2−mg−Fsurface 1=−4S(l+y)tan⁡θ2−mg−4Stan⁡θ2ya=−4Stan⁡θ2my=−4Stan⁡θ24Sltan⁡θ2gyd2ydt2=−gly∴T=2πlg

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