Probably a good thing the forum was down as I contemplated a response
Hmmm, this is a lot more interesting than the flat tire question!
As for the second cart with the cute animal toys (that's what our flat tire story was missing!), I just thought of a very simple analogy. Picture a 4 foot long stick, with an axle at the 1 foot point, so it is just a 3:1 lever. Now, holding it vertically by the axle, you could push on the short end, and the long end would move in the opposite direction but 3x faster.
That would not make a very compelling video, because it is so simple and transparent. But I think that that is all he has really accomplished with the wheels and cart, right? The different sized wheels just give more speed to the small wheels compared to the big wheels, and change the direction of motion. But it is very clever and well done - I couldn't really predict what would happen.
I'll ponder that propeller cart some more.... but I'm thinking it may be similar - with the propeller versus still air sort of forming a fulcrum point that the cart can move around, but maybe not....
Something tells me that the two Sierra Nevada Pale Ales (cloned homebrews) I have chilling are *not* going to help me figure this out
-ERD50