Damped harmonic oscillator consists of a block `(m=2kg)` a spring `(k=8 pi^(2)N/m)` and a damping force `(F=-bv)` .Initially it oscillates with an amplitude of `25cm`. Because of the damping the amplitude falls to three-fourth of this initial value at the completion of four oscillations.What is the value of b ? (Assume small damping and take : `ln((3)/(4))=-0.28`