When the upward and downward forces on a falling object are equal, the object reaches terminal velocity.

Here's why:

* Downward Force: Gravity pulls the object downward, causing it to accelerate.

* Upward Force: As the object falls, it encounters air resistance, pushing upward against its motion. This resistance increases as the object's speed increases.

* Equilibrium: When the downward force of gravity is exactly balanced by the upward force of air resistance, the object stops accelerating. It continues to fall, but at a constant speed called terminal velocity.

Important Note: Terminal velocity is not a fixed speed. It depends on the object's shape, size, and mass, as well as the density of the air. A heavier object will have a higher terminal velocity than a lighter object of the same shape.