The acceleration of gravity is 9.8 m/s². That means that when a rock
is falling, its speed keeps increasing, and is always 9.8 m/s faster than
it was 1 second earlier.
If you just let it roll out of your hand, then it starts from zero speed, and
after 4 seconds, its speed grown to (4 sec) x (9.8 m/s²) = 39.2 m/s .
If there's no wind blowing it sideways, then its velocity is
39.2 m/s straight down.
Its AVERAGE speed during the 4 seconds is (1/2) (0 + 39.2 m/) = 19.6 m/s .
Falling for 4 seconds at an average speed of 19.6 m/s,
it falls (4 sec) x (19.6 m/s) = 78.4 m .
If there's no wind blowing it sideways, then its displacement is
78.4m straight down.