Consider four pendulums tied to a string and suspended from a rigid support. Let the lengths of pendulum A and C be identical while pendulum B and D have different lengths. When pendulum A is displaced, it undergoes free oscillation with a natural frequency. The natural frequency is the frequency at which the pendulum oscillates naturally in the absence of any damping or driving force. The energy from pendulum A is transferred to the other pendulums forcing them to oscillate with a driving frequency. These resulting oscillations due to the external driving force exerted by pendulum A are known as forced oscillations. The driving force acting on the pendulums can be represented by the periodically varying force with amplitude F0 and the driving angular frequency. By adding the obtained driving force to the force equations of the damped harmonic oscillator, the forced oscillator's equation of motion is obtained.