I was driving back from dancing tonight and I noticed that the hubcaps on the car next to me appeared to be spinning slowly backwards. I figured this was probably aliasing due to 60Hz flicker from the streetlights. After a brief moment of confusion (no way were the guy’s tires spinning close to 60 times a second!), I realized that the hubcaps probably had some degree of rotational symmetry, and it wouldn’t be hard to figure out how much.

I know that we were both going around 50mph, maybe a little slower. I’m assuming that our wheels were about the same size; mine are 2.00 feet in diameter. That’s about a 6.28 ft circumference. 50 miles/hour = 264000 ft/hr = 73.3 ft/sec. So my wheels (and presumably the other guy’s as well) were rotating about 11.7 times/sec. If the wheels appeared to rotating slowly backward, the effective frequency must have been just a little under 60Hz, so I’m guessing that the other guy’s hubcaps had 5-fold rotational symmetry.

Who ever said math isn’t important?