A simple pendulum is arranged using a small metallic bob of mass 'm'and a light rubber cord of length 'L' (on suspending the bob), area of cross section 'A' and Young's modulus 'Y'. [One should use inextensible cord only for simple pendulum!]. When this unconventional pendulum is at rest in its mean position, the bob is pulled slightly down and is released. Then, the period of the vertical oscillation of the bob is (assuming that the size of the bob is negligible compared to the length of the cord):