An object reaches terminal velocity when the force of gravity pulling it down is balanced by the force of air resistance pushing it up. This means the object is no longer accelerating and is falling at a constant speed.

Here's a breakdown:

* Gravity: Gravity pulls the object downwards, causing it to accelerate.

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

* Terminal Velocity: When the force of air resistance equals the force of gravity, the object stops accelerating. The speed at which this happens is called terminal velocity.

Factors affecting terminal velocity:

* Mass of the object: Heavier objects have a greater gravitational force acting on them, so they reach a higher terminal velocity.

* Shape and size of the object: Objects with larger surface areas experience more air resistance, resulting in a lower terminal velocity.

* Density of the air: Denser air provides more resistance, reducing terminal velocity.

Important Note: Terminal velocity is not a fixed value for a given object. It changes based on the factors mentioned above.