teaching physics to high school students - a true story

I've told this story before. But it's good enough to immortalize.

I used to teach high school physics (in addition to six other subjects every day) at a private boarding school. It was a pretty rotten job for a number of reasons, though the students themselves were pretty funny. The vast majority were from mainland China, though we had a couple Koreans, a Russian, a Dane, and two Americans.

Even though nearly everyone was not from "here", certain aspects of humor seemed universal.

I was trying to teach some of my students about rotational motion. This is often cited as the most difficult part of mechanics, and, for some, the most difficult part of the entire course. It was an algebra-based course, so it didn't involve cool integrals of nonuniform shapes or utilize nonuniform densities. Everything was pretty boring -- a rectangular prism, a sphere, a rod, a disk, a hoop, a sphere, and, occasionally, a hollow sphere. (That reminds me - I should look into calculations for a right cone. That would probably blow my mind.)

I was explaining the different rates of speed achieved by different shapes. Because different objects have different moments of inertia (an expression for how the mass is distributed throughout the shape that affects rotation), they split their energy in different ways between translational motion (moving in linear direction) and rotational motion (spinning). So a disk, a sphere, and a hoop, all with the same radius and mass, will move down a ramp at different speeds because they have different moments of inertia.

I drew something like this:

Needless to say, as a first-year teacher, I was unprepared for the howls of laughter. I looked at this and saw the finishing order for a sphere (gray), a disk/cylinder/can, and a hoop.

They, of course, saw a cock and balls.

Atwood's machine was also troubling:

Anyway, I ended up putting the question involving rotational motion down a ramp for a sphere, a cylinder, and a hoop, all of mass M and radius R, on a quiz. One particularly lazy/uninspired student answered this question, and this one only. He answered it by drawing the picture above, eight times in the space provided for an answer.

What was I to do? He was technically right. So I gave him full credit. Besides, that was the only thing he had written on the quiz, and I didn't feel like handing out a zero that day. He still failed the quiz and the class.

The moral of the story: high school boys are very much the same across cultures.