Let block is displaced through x m, then weight of displaced water or upthrust (upwards) =−Axρg Where, A is area of a cross-section of the block and ρ is density. This must be equal to force (= ma) applied wherem is mass of the block and a is acceleration. ∴ ma=−Axρg ∴ a=−
Aρg
m
x=−ω2x This is equation of SHM. The time period of oscillation, T=