From rest with no air and constant g, h = gt^2/2, t = sqrt(2h/g), and v = sqrt(2gh). Use downward magnitudes for these formulas. The height is a model input, never an experimental instruction.
Worked example
For h = 20 m and g = 10 m/s^2, t = sqrt(4) = 2 s and v = sqrt(400) = 20 m/s. The average is 20/2 = 10 m/s.
With no air and constant g of 10 meters per second squared, an object released from rest falls 5 meters in its first second and 15 more meters in its second second. Total distance is 20 meters.
