Flipping Tennis Rackets

When I teach non-science majors, I worry more about getting across to them the wonder that is science, rather than lots of details or explanations. I recently taught a class with mostly art, film, and literature majors, and I think I did a decent job of communicating to them that I do science/physics for pretty much the same reasons they do what they do...it's wonderful, amazing, interesting, and beautiful. I think many are really surprised by this.

In any case, one of the really cool things showed up when I was a freshman in college. For some reason, most of my physics classes were at 8am. I didn't have any problem waking up that early, I just couldn't stay up! Fortunately, high school had taught me to sleep with my eyes open.

So I'm half asleep, and my internal clock, detecting the end is near, is slowly ramping up my consciousness, and then I start hearing what the whole lecture has been about....rotational dynamics and instability. The board is filled with equations and steps, and I realize we've explained something that's been an oddity to me for years.

Tennis players are always flipping their tennis rackets. When the other person serves, I spin my racket in my hand (along the axis through the handle). If I have to wait for something, I may spin the racket edge on, so you don't see the face. But if I'm really killing time, I point the racket out like I'm holding a tray, and then I flick it in the air. Almost all of the time, the racket then does a spin, like a high diver doing a twist. If I try real hard, I can usually flip it without the twist but it's hard. Why? Well, using just some chalk on a board, we had explained why a flip about (what turns out to be) the intermediate axis is unstable!

A more complete explanation of this effect can be found at Physics Central, but nicer videos (but with a board, not a tennis racket) is up here (the third axis is the one with the instability).

To a non-physicist this result may not elicit much excitement, and may even seem trivial. But for me, ...well, it's hard to explain, but it's powerful. It boils down to power. Physicists are so powerful, we have control. Like the Connecticut Yankee in King Arthur's Court, we have power because we have knowledge and understanding. It's a bit like a kid with one of those chemistry sets, or the adult who dreams of having all those power tools with lasers.