How can a soccer ball curve around a defender?

A free kick can begin toward one side of a defender and bend toward the goal. After the ball leaves the foot, the player cannot keep pushing it. The continuing sideways push comes from the air interacting with a spinning ball.

An off-center kick gives the ball rotation as well as forward motion. Near its surface, air is dragged along by that rotation. This changes the surrounding flow and pressure pattern. The ball deflects air, and the air exerts a sideways force on the ball. This spin-related aerodynamic effect is called the Magnus effect.

For a ball spinning about a roughly vertical axis, the added force points mainly sideways. It changes the direction of travel little by little, making a curve. Gravity still pulls downward, and drag slows forward motion. The path is therefore not a perfect flat circular arc.

The amount of bend depends on speed, spin, air density, and the ball’s surface. Panel seams influence how air flows and separates from the surface. A kick with little spin may behave differently because its wake can change irregularly. A curling shot is an encounter between a moving surface and invisible moving air.