Ping Pong

When I was in a teenager, I played a lot of ping pong. It was the sport that I played best (which isn’t saying much) and loved most (except for baseball). Dad bought a ping-pong table for the family when I was young, and he was my first opponent. As I grew up, I spent hours practicing with friends, perfecting our serves, slices, and slams. In high school, my friend Terry Fife and I played hundreds of games, and we were evenly matched. The psychological warfare during those battles was fierce.

I continued playing ping pong in college. I bought one of those paddles that has foam under the rubber so the ball stays in contact with the surface for a long time, letting you impart more spin. During my first two years at the University of Kansas, I studied but also spent a lot of time playing ping pong at the dorm. I was better than most of the guys, although my friend Son Do could beat me, much to my chagrin. Too many evenings were squandered playing cards or ping pong. Only during my last couple years at KU did I get serious about physics.

Life in Moving Fluids,
by Steven Vogel.

Understanding ping pong requires us to discuss the Magnus effect, which Steven Vogel illustrates by considering a spinning cylinder in a flowing fluid. In Life in Moving Fluids—a book often cited in Intermediate Physics for Medicine and Biology—Vogel writes

A look at the resulting streamlines (Figure 10.12 [a modified version of which is shown below]) clarifies what’s happening in this superposition of rotation and translation of a cylinder. On one side of the cylinder the two motions in the fluid oppose one another, so the velocities are lower and the streamlines are farther apart. On the other side, the motions are additive, velocities are increased, and the streamlines are closer together. By Bernoulli’s principle pressure will be elevated on the side where flow speeds are lower and will be reduced on the side where the speeds are higher. Thus a net pressure or force will act in a direction normal to the free-speed flow—in short, lift…

Figure 10.12. If a solid body such as a cylinder rotates as it translates through a fluid, the resulting asymmetry of flow generates a force normal to the free-stream flow. We call this force lift. Adapted from Life in Moving Fluids by Steven Vogel.

This phenomenon, the lift of a rotating cylinder moving through a fluid, is called the “Magnus effect,” after H. G. Magnus (1802-1870).

The Magnus effect (at a little lower intensity) works for spheres as well as for cylinders. It’s a really big deal in sports in which spheres are thrown, hit, or otherwise put into motion since (except for a golf slice) a confusingly nonstraight course is distinctly meritorious. Two pleasant books on such contemporary compulsions are Sport Science, by P. J. Brancazio (1984), with good references, and The Physics of Baseball, by R. K. Adair (1990), with more on the Magnus effect specifically.

Me playing ping pong at the
University of Kansas, circa 1980.

Vogel’s description of the Magnus effect is clear and interesting, but I think it’s not very relevant. In ping pong, the goal of spin is not to make the ball follow a curved path through the air, but instead is to make the ball leave your opponent’s paddle traveling in the wrong direction! After putting backspin on the ball, I loved to see Terry Fife’s return dive into the net. And I enjoyed nothing more than giving the ball some sidespin, and then watching Son Do’s next hit miss the table on the right. Those were the days!