Problem 4.20

A ball is suspended by a thread of length $l$ at the point $O$ on the wall, forming a small angle $\alpha$ with the vertical (Fig. 4.1 ). Then the thread with the ball was deviated through a small angle $\beta(\beta>\alpha)$ and set free. Assuming the collision of the ball against the wall to be perfectly elastic, find the oscillation period of such a pendulum.