ma=m*g*sinA
When a is small (tends to 0), Sina is about equal to a.
m*a=m*g*A……( 1)
The differential equation corresponding to equation (1) is a second-order ordinary differential equation whose solution
s = c 1 * sin[sqrt(g/L)* A+B]+C2
(s represents the displacement from the central position C 1, C2, b, determined by the initial rod)
So this cycle
t = 2 * pi/(sqrt(g/L))= 2 * pi * sqrt(L/g)