Under the Ruler Faster than the Ruler

I was just talking to Michael Weisberg, who is also visiting ANU currently, and he pointed me to this video showing a counterintuitive physics demonstration. I had seen the video before, so we started discussing how it might work. He pointed me to the explanation videos that the author of that video made, but they don’t really clarify things very much. When we tried to work it out ourselves, we came to the conclusion that it had to be impossible (unless there was slippage between the wheels or one of the surfaces) – until I realized one feature of the cart that I hadn’t noticed before.