Why Curveballs Curve

The simulation shows an overhead view of a ball moving through the air. When a ball is not rotating, air flows around it equally on both sides. When the ball rotates, the picture is no longer symmetric. On one side the air goes faster and on the other side it goes slower. This gives rise to a pressure difference, and a net force making the ball curve. In this case the lower pressure region is above the ball, so the ball curves in that direction.