Simple pendulum
= Simple pendulum
{title2=$\ddot\theta+(g/\ell)\sin\theta=0$}
A point mass constrained to a fixed-length massless arm under uniform gravitational <acceleration> is a simple pendulum. With no drag its angular equation is $\ddot\theta+(g/\ell)\sin\theta=0$. The <small-angle approximation> yields <simple harmonic motion>, whereas the full restoring force is nonlinear. Adding drag gives a <damped pendulum>.