9.81

9.81, this is the gravitational acceleration factor on the Earth.

This means that, ignoring air resistance, an object falling freely near the Earth's surface increases its velocity by 9.81 m/s (32.2 ft/s or 22 mph) for each second of its descent.

But there is air resistance in actual world, so the velocity becomes constant value when the air resistance becomes equivalent to gravity; F = mg, m is the mass of the object, and F is the skin friction or interface drag.