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
-- for now, it will be sufficient simply to stop the
calculation when x becomes negative