The acceleration of gravity here on our home-planet is 9.8 m/s².
That means that at any time during the fall, a falling object is falling
9.8 m/s faster than it was 1 second earlier.
Looking at it another way, the speed of any falling object increases
by 9.8 m/s every second while it falls.
If Superman fell for 7.8 seconds, then his falling speed gained
(7.8) x (9.8 m/s) = 76.44 m/s
during the time he spent falling.
Since his falling speed was zero when he 'stepped' off of the top,
he hit the ground at 76.44 m/s .
Note:
That's about 170 miles per hour.
I'll bet he left one serious crater !
