Air is thin, but a fast ball has to shove a lot of it aside. The ball feels a drag force that points against its motion:

F_D = ½ ρ C_D A v²

• ρ is the air density, about 1.2 kg/m³ at sea level.
• A is the frontal area of the ball: for a football of 22 cm diameter, A = π r² ≈ 0.038 m².
• C_D is the drag coefficient, a pure number that describes how “draggy” the shape and surface are. For a football at free-kick speeds a value near 0.25 is typical (it is noticeably higher at low speeds, and it changes with speed, as World 2 explains).
• v is the speed relative to the air.