Pendulum with a modulated length. Part I. Parametric resonance

Abstract

Parametric excitation of a rigid planar pendulum caused by a square-wave modulation of its length is investigated both analytically and with the help of computer simulations. Such a system can be considered as a simple model of a playground swing. Transformations of energy, the threshold of excitation and other characteristics of parametric resonance are discussed in detail. The role of nonlinear properties of the pendulum in restricting the resonant swinging is emphasized. The feedback providing optimal control of pumping and damping is analyzed. Phase locking between the drive and the pendulum at large amplitudes and the phenomenon of parametric autoresonance are discussed.