Pendulum Phase Plane Applet
This applet draws numerical aproximations to the phase curves for
the system of differential equations

This system is equivalent to the second-order equation
the equation of a pendulum.
Clicking on a any point in the applet begins a phase
curve at that point. The `Continue` button will continue a curve
from its final point. You may choose to use either Euler's method (drawn
in red),
a Runge-Kutta method of order 2 (drawn in green), or a Runge-Kutta method
of order 4 (drawn in blue).