The distance covered by a free-falling object in 'T' seconds after it's dropped is
D = (1/2) x (gravity acceleration) x (T²)
Gravity acceleration on Earth is 9.8 m/s². or 32.2 feet/sec² .
20-ft = (1/2) (16.1) (T²)
20 = 8.05 T²
T = √(20/8.05) = 1.576 seconds .
