Explanation and answer:
To solve this problem, we use the kinematics equation:
v^2-u^2=2aS .........................(1)
where
assuming the positive direction is downwards)
v = final velocity (when it reaches ground)
u = 0 = initial velocity (when it was dropped, or released, with zero vertical velocity).
a = 9.81 m/s^2 = acceleration due to gravity (on earth's surface)
S = 90m = distance travelled.
Substitute values into (1)
v^2 - 0^2 = 2*(9.81)*90
v^2 = 1765.8 m^2/s^2
v = sqrt(1765.8) m/s
= 42.02 m/s
= 42 m/s (to two significant figures).
