In this case the system is configured by a mass linked to a fixed mass at the top (invisible) by means of a non rigid rope (spring). External gravity is set and the mass has been given a charge in order to be able to apply an external sinusoidal force by means of an electric field. The trace of the oscillating mass is shown.
The natural period for this system is 2psqrt(l/g), being l the length of the pendulum and g the value of the acceleration of gravity. The period, for an ideal pendulum, does not depend on the mass. (But do not try to check this now, because changing the mass produces unwanted vibrations due to the elasticity of the "rope").