Hi. Imagine an imaginary car with a huge windmill on top connected to its enginator. Assume windmills and enginator are 100% efficient (not possible, I know, but it's a thought experiment). Perpetual motion? What exactly causes it to fail? (I.e. why won't it self-accelerate indefinitely) I'd be more interested in a hint, than a full answer. And by "what exactly causes it to fail", I'm _not_ looking for answers like "thermodynamics says there's no PMMs", etc. but the precise reasons (or a hint as to them, actually) why this particular scheme fails.