Objects do reach a constant terminal velocity when falling through a medium, such as air. However, it's important to distinguish between objects falling in a vacuum and objects falling through a medium.
In a vacuum, where there is no air resistance, objects of different masses fall at the same rate, often referred to as the acceleration due to gravity, which is approximately 9.8 meters per second squared on the surface of the Earth. This is because the only force acting on the object is gravity, and mass cancels out in the equation for acceleration. Therefore, in a vacuum, objects do not gain velocity as they fall; they experience a constant acceleration due to gravity.
On the other hand, when objects fall through a medium like air, they experience air resistance, which opposes their motion. Initially, as an object begins to fall, its velocity increases because the force of gravity is greater than the opposing force of air resistance. However, as the object accelerates, the air resistance also increases. Eventually, a point is reached where the force of air resistance becomes equal to the force of gravity. At this point, the net force on the object becomes zero, resulting in no further acceleration. The object then reaches a constant velocity known as the terminal velocity.
The terminal velocity depends on the specific characteristics of the falling object, such as its shape, surface area, and mass, as well as the properties of the medium through which it is falling. Objects with larger surface areas or less aerodynamic shapes experience greater air resistance and therefore reach their terminal velocity more quickly. In contrast, objects with smaller surface areas or more streamlined shapes experience less air resistance and take longer to reach their terminal velocity.
To summarize, in a vacuum, objects fall with a constant acceleration due to gravity. In a medium with air resistance, objects initially gain velocity but eventually reach a constant terminal velocity where the force of gravity is balanced by the force of air resistance.