The Position–Velocity Phase PlaneDamped Nonlinear VibrationsThe Nonlinear PendulumPeriod of Undamped OscillationDamped Pendulum Oscillations9.4 ProblemsCritical Points for Damped PendulumCritical Points for Mass-Spring SystemCritical Points for Physical SystemsPeriod of Oscillation9.4 Application The Rayleigh, van der Pol, and FitzHugh-Nagumo EquationsRayleigh’s EquationVan der Pol’s EquationThe FitzHugh-Nagumo Equations