136. When you suspended the Ping-Pong ball in the stream of air from the pipe, why did the ball spin? The same thing happened to the two flat pieces of plastic that were held together when air flowed out between them.
The Ping-Pong ball spun because the viscous drag forces it experienced weren't equal on all sides. As we'll see shortly, there are a variety of different drag forces and they can act at different locations on an object. In the case of viscous drag, it acts locally at each point where air slides across the surface of the object. Since the airflow from the pipe wasn't perfectly uniform, the air swept past the ball faster in some places than it did in others. These differences in airspeed became most significant when the ball began to drift away from the airstream—the sudden increase in airspeed on the side of the ball nearest the center of the airstream is what created the low pressure that allowed the surrounding air to push the ball back toward the center of the airstream. But minor differences in airspeed also exerted unbalanced torques on the ball and caused it to spin. Similar flow imperfections between the two plates created differences in viscous drag and exerted torques on the two plates. That's why they began to spin around slightly.