Consider the turntable of an old record player. Or equivalently, a CD affixed to a player so that it may spin freely. We’ll pretend there’s no friction, though as always reality will manage to generate some. Now stretch your imagination a bit further and imagine that you shrink yourself down to a height of about 6 inches or so and stand on the turntable/CD. You look around, and having decided that now’s as good a time as any to get started on your exercise routine, you start walking laps around the disc.
Conserving angular momentum, the surface will begin to rotate under you in the opposite direction – though not in general at the same speed. From the viewpoint considering forces, the forward force the floor applies to your foot is matched by the backwards force your foot applies to the floor. When you stop, essentially the same thing happens in the opposite direction.
I’m deliberately skipping all the mathematics here. This is to build intuition.
Building on this force approach to angular momentum, is the following XKCD scheme viable?
I am more than certain that this has been beaten to death elsewhere on the web, as Randall Monroe has one of the most erudite fanbases on the internet and probably himself has at least three 9s in his intelligence percentile. Nonetheless, I’d like to give y’all a shot at it. Even the triple 9s can make mistakes…