Let the ball fall a distance s from the initial height.
Initial velocity u=0
Acceleration a=g=10 m/s2
Given that the magnitude of velocity is equal to the magnitude of acceleration due to gravity, we have v=10 m/s.
Using the equation of motion v2=u2+2gs:
(10)2=02+2×10×s
100=20s
s=5 m
The distance fallen by the ball is 5 m.
The height above the ground is h=H−s
h=18−5=13 m
Answer: 13