* Air resistance: As an object falls, it experiences air resistance, which is a force that opposes its motion. This force increases as the object's speed increases.

* Terminal velocity: Eventually, the force of air resistance becomes equal to the force of gravity pulling the object down. At this point, the object stops accelerating and falls at a constant speed called terminal velocity.

Therefore, the speed of a falling object increases at a decreasing rate until it reaches terminal velocity.

In a vacuum, where there is no air resistance, the speed of a falling object does increase at a constant rate (due to the constant acceleration of gravity).