Understanding Forces and the Magnus Effect A skilled footballer can kick a ball so that it curves around a defensive wall and towards the goal. The ball is not simply following the path it had when it left the player's foot. Forces continue acting on it while it travels through the air. Gravity pulls the ball downward, while aerodynamic drag acts against its motion. If the ball is spinning, another aerodynamic force can act sideways. A spinning ball changes the airflow around its surface. The interaction between the rotating surface, surrounding air and the wake behind the ball creates an uneven pressure distribution. This can produce a sideways force known as the Magnus effect. The direction of this force depends on the direction of spin. By striking the ball away from its centre, a player can give it both forward velocity and rotation. The ball's final path depends on its initial speed, launch angle, spin rate and the aerodynamic forces acting on it. Its motion is therefore much more complicated than the ideal projectile paths sometimes studied in basic physics. Similar effects occur in tennis, cricket, baseball and golf. Athletes deliberately control spin to alter the motion of balls through the air. A curling free kick is therefore a visible demonstration of forces continuously changing an object's velocity after it has left the person who launched it.