Let the cube be at a depth x from the equilibrium position. Force acting on the cube = up thrust on the portion of length x. F=−ρl2xg[∴ mass density X volume ] ....... (i) Clearly F∝−x, Hence it is a SHM. Equation of SHM is F=−kx .....(ii) Comparing equation (i) and (ii) we have k=ρl2g Now, Time period, T=2π√
m
k
⇒T=2π√
l3d
ρl2g
=2π√
ld
ρg
Comparing the above equation with a=−ω2x, we get ∴ω=√