In order to determine the maximum initial velocity, use the following formula for the maximum height that an object reaches:
where,
vo: maximum intial velocity = ?
g: gravitational acceleration constant = 9.8 m/s^2
h: maximum height = 20 m - 1.5 m = 18.5 m
Look that h has been taken regarding with where the ball was thrown.
Solve for vo in the previous formula and replace the values of the oher parameters, as follow:
Hence, the maximum initial velocity should be approximately 19.04 m/s