I have a pet idiot, you know the type. "Perpetual motion is the future, we dont understand everything!" Well we've been discussing gravity, (and magnets, oh how he goes on about magnets). He doesnt seem to be able to see the difference between force and energy. I've finally gotten him to understand that an apple dropping from 1 meter and accelerating is not the creation of energy as energy had to be put in to get the apple to the height in the first place. What I cant explain to him is the following: A rock is traveling through space heading towards the planet Earth. Its traveling slowly relative to the earth, only about 50kph. As it's heading towards the earth, then eventually it encounters the earths gravity. It accelerates towards the earth and eventually impacts. When it impacts it's going much faster than 50kph. Does that additional velocity mean more energy, and if so where did it come from. Please dont let my idiot carry on like this. He's looking so smug right now, because I dont know the answer.