-- At the end of 18sec, the stone's velocity is (9.8m/s² x 18s) = 176.4m/s downward.
-- Starting at 0 m/s and ending at 176.4m/s, its average velocity is 88.2m/s downward.
-- Falling for 18s at an average speed of 88.2m/s, it falls (88.2m/s x 18s) = 1,587.6 m .
-- Its displacement is 1,587.6 meters downward.
