NCERT Solutions
Class 11 Physics
Oscillations

Q. 13.17
A cylindrical piece of cork of density of base area A and height h floats in a liquid of density ρl. The cork is depressed slightly and then released. Show that the cork oscillates up and down simple harmonically with a period
T= 2π √ (hρl) / (gρl) where ρ is the density of cork. (Ignore damping due to viscosity of the liquid).
Base area of the cork = A
Height of the cork = h
Density of the liquid = ρ1
Density of the cork = ρ
In equilibrium:
Weight of the cork = Weight of the liquid displaced by the floating cork
Let the cork be depressed slightly by x. As a result, some extra water of a certain volume is displaced. Hence, an extra up-thrust acts upward and provides the restoring force to the cork.
Up-thrust = Restoring force, F = Weight of the extra water displaced
F = – (Volume × Density × g)
Volume = (Area × Distance through which the cork is depressed)
Volume = Ax
∴ F = – (A x ρ1 g)..... (i)
According to the force law:
F = kx
k = (F/x) where, k is constant
k = (F/x)
= - (Aρ1 g).... (ii)
The time period of the oscillations of the cork:
T = 2π√ (m/k) .... (iii)
Where,
m = Mass of the cork
= (Volume of the cork × Density)
= (Base area of the cork × Height of the cork × Density of the cork) = (Ahρ)
Hence, the expression for the time period becomes:
T=2π√ (Ahρ)/ (Ahρ1g)
T = 2π√ (hρ)/ (ρ1g)