$T = 2\pi \sqrt {\frac{{{\rm{displacement}}}}{{{\rm{acceleration}}}}} $
$ = 2\pi \sqrt {\frac{x}{{\frac{{gx}}{{(R - t)}}}}} = 2\pi \sqrt {\frac{{R - r}}{g}} $

Find time after which to the energy will become half of initial maximum value in damped force oscillation.
