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.