The amplitude of a simple pendulum, oscillating in air with a small spherical bob, decreases from 10 cm to 8 cm in 40 seconds. Assuming that Stokes…

The amplitude of a simple pendulum, oscillating in air with a small spherical bob, decreases from $\displaystyle 10$ cm to $\displaystyle 8$ cm in $\displaystyle 40$ seconds. Assuming that Stokes law is valid, and ratio of the coefficient of viscosity of air to that of carbon dioxide is $\displaystyle 1.3$, the time in which amplitude of this pendulum will reduce from $\displaystyle 10$ cm to $\displaystyle 5$ cm in carbondioxide will be close to ( $\displaystyle \ln 5=1.601$, ln $\displaystyle 2=0.693$ ).