-- Gravity adds 9.8 m/s to his falling speed every second.
After 2 seconds, his falling speed is (2 x 9.8) = 19.6 m/s .
-- His initial speed ... as he steps off the roof ... was zero.
His final speed, at the safety net, is 19.6 m/s.
His average speed all the way down is (1/2) (0 + 19.6) = 9.8 m/s .
-- Falling for 2 seconds at an average speed of 9.8 m/s,
he covers a vertical distance of (2 x 9.8) = 19.6 meters.
Cool discovery:
After falling from rest for 2 seconds, the speed is the same number as
the acceleration, and the distance fallen is double the acceleration.
In ANY system of units.
(But the units of the three numbers are different.)
