Does the legal definition include a sled going down a hill? If sleds, then skis aren't much of a stretch. And if skis, then shoes aren't much of a stretch.

When I was reading the dictionary definition, I got the sense that gravity isn't eligible as the motive force, and instead it would need to be someone exerting themselves (like pulling a wagon) or using stored energy (like an automobile). But sliding down a hill by gravity is indeed using stored energy, so I'd think that counts!