implement the program described in the "simple
example," with dt = 0.1
run it for x0 = 1,
v0 = 4, a = -0.5
plot the trajectory (t, x) of the particle
until it reaches x < 0
on the same graph plot the analytic trajectory x(t)
compare the time to reach x = 0 with the
analytic (Physics I) formula -- stop the calculation
when x becomes negative, then use linear
interpolation to refine the numerical result before
making the comparison