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A uniform cylinder of length L, mass M and cross-sectional area A hangs vertically from a fixed point by a massless spring of constant k, and at equilibrium it is half submerged in a liquid of density σ. Given a small downward push, it oscillates vertically with small amplitude. Its time period T is

Asked in JEE Main Online 2013 · Floating bodies and gas columns

Answer: (4) 2π[M/(k+Aσ g)]^1/2

Step-by-step solution

A downward displacement x stretches the spring, giving -kx, and submerges an extra volume Ax, giving a buoyant restoring force -Aσ gx.

The two act together, so k_eff=k+Aσ g.

T=2π√M/(k+Aσ g) exactly, as long as the cylinder stays partly submerged.

Why the other options are wrong

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