Spin and lift

For convenience, I’ll stick with baseball pitches for this section to describe how ball’s move when spin is introduced. As everyone knows, you can make a ball curve in the air by releasing it such that it spins on an axis perpendicular to its direction of motion. The exact axis governs which way the ball will curve.

There are two lift forces that can occur when a ball is spinning backwards relative to its direction of motion (backspin):

•  The Magnus force

•  The wake deflection force.

The Magnus force

The Magnus force is the sole lift force during laminar airflow. It occurs because the side of the ball that is spinning in the direction of the airflow actually carries some of the surrounding air with it. Another way of saying this is that this surrounding air bends towards the ball, and the air surrounding the side of the ball spinning in the opposite direction of motion bends away from the ball. Remember from earlier that the air curving toward the ball yields a lower pressure. Since we are talking about backspin, the air moving with the spin direction is the top of the ball. Therefore, the top of the ball experiences a pressure less than atmospheric and the bottom of the ball, where the air is curving away, experiences greater than atmospheric pressure, so the ball experiences a net upward force. We call this force lift.

The wake deflection force

During turbulent airflow, the wake deflection force is present along with the Magnus force. The wake deflection force occurs when its rapid spin deforms the wake. The spinning ball's moving surface pushes the airstreams with a viscous force. Thus, with backspin, the wake is pushed at an angle to the other side. The faster the spin, the greater these two forces, and the direction of the spin axis determines the direction of the lift force being exerted.

Tennis, table tennis, soccer, baseball, and badminton are just a few examples of how crucial aerodynamics is, and how man's understanding of the subject has aided not only with airplanes, space flight, and racing, but also with leisure and competitive activities that we so enjoy.

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