--This example simulates a falling object with air friction proportional to the square of its velocity. The technique used is inefficient compared with fancier methods for numerical solutions of differential equations, but it is very easy to program. See example "projectile" for a similar problem in two dimensions.
-- stepto(0) sets up initial conditions. Any other stop time runs the simulation from where it left off up until the new stop time (within dt). Since the plot calls for points in order, the simulation does not need to start over for each plotted point.