The gain in velocity per second for a freely falling object is 9.8 m/s², which is the acceleration due to gravity on Earth. This means that for every second an object falls, its velocity increases by 9.8 meters per second.

Here's why:

* Gravity: Earth exerts a force of gravity on all objects near its surface.

* Acceleration: This force causes a constant acceleration, meaning the object's velocity changes at a constant rate.

* Free Fall: "Free fall" means the only force acting on the object is gravity. Air resistance is ignored.

Important Notes:

* This value (9.8 m/s²) is an approximation. The actual acceleration due to gravity varies slightly depending on location.

* This is the *acceleration*, not the *velocity*. Velocity is how fast something is moving, while acceleration is how much its velocity changes over time.