When an object free-falls through vacuum (no air) pulled by gravity,
there's no limit to the speed it can reach. It keeps falling faster and
faster until it hits something.
When the object is falling through air, the effect of air resistance
places a limit on the maximum speed it can reach. It falls faster
and faster until it reaches that speed. After that, it stops accelerating,
and falls the rest of the way at a constant speed.
