Title:Decay of amplitude of a harmonic oscillator with weak nonlinear damping

Abstract:We demonstrate how to derive approximate expressions for the amplitude decay of a weakly damped harmonic oscillator in case of a damping force with constant magnitude (sliding friction) and in case of a damping force quadratic in velocity (air resistance), without solving the associated equations of motion. This is achieved using a basic understanding of the undamped harmonic oscillator and the connection between the damping force's power and the energy dissipation rate. Our approach is based on adapting the trick of adding the energy dissipation rates corresponding to two specific pairs of initial conditions, which was recently used to derive the exponential decay of the amplitude in case of viscous damping, to these two types of damping. We obtain two first-order differential equations from which we get the time-dependent amplitudes corresponding to both damping forces. By comparing our approximate solutions with the exact solutions in the case of sliding friction and with the approximate solutions given by a another well-known method in the case of air resistance, we find that our solutions describe well the dynamics of the oscillator in the regime of weak damping with these two forces. The physical concepts and mathematical techniques we employ are well-known to first-year undergraduates.