A simple pendulum is a setup in which a heavy point mass is suspended from rigid support by a weightless, inextensible, and completely flexible string.

Expression for the time period:

For an angular momentum,  sin θ, so that

F = -mgsin θ

   = -mgθ

   = -( mg/l )y = -Ky

The time period of the simple pendulum is T=2π√L/g  since Y = lθ. Only when the length of a simple pendulum () is small in comparison to the radius of the earth is this equation valid.

If a simple density rho 

(ρ), the pendulum is constructed to swing in a density rho(ρ) liquid, its time period will rise in comparison to that of air, as shown by:

 T=2π√L/(1- σ/ρ)

If the bob of a simple pendulum has positive charge q and the pendulum is placed in a uniform electric field E which is in vertically downward direction then the time period decreases.

 T=2π√L/g+qe/m