From 18 m height above the ground a ball is dropped from rest. The...

Let the ball fall a distance $s$ from the initial height.

Initial velocity $u = 0$

Acceleration $a = g = 10$ m/s$^2$

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 $v^2 = u^2 + 2gs$:

$(10)^2 = 0^2 + 2 \times 10 \times s$

$100 = 20s$

$s = 5$ 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: $13$