Thanks guys. Gives me some ammo for him.
Dumbing it down a little I'm going to hit him with the K+P=constant, and that gravity doesn't create energy, it converts it. Not that it will convince him of course.
I have a pet idiot, you know the type. "Perpetual motion is the future, we don't understand everything!"
Well we've been discussing gravity, (and magnets, oh how he goes on about magnets).
He doesn't seem to be able to see the difference between force and energy.
I've finally gotten him to...