In December 2008, one of my favorite students emailed me an interesting physics problem:
Suppose you’re in an airport and you need to get from point A to point B. For part of the way, there’s a moving walkway; the rest of the way, there isn’t. You walk at your normal speed, both on the moving walkway and on the floor. The questions are:
1. If you have to stop to tie your shoe, should you do it on the floor or on the walkway? (Your goal is to make the trip in as short a time as possible.)
2. If you can run for a short, fixed period of time, do you save more time by running on the walkway or on the floor?
3. How do the answers to (1) and (2) change if you take special relativity into account?
I wrote up a solution, which I’m very proud of. Here it is: