Our high school student now has a question about potential energy due to gravity. she says, "i don't get the formula pe = mgh. it seems i can change the amount of potential energy something has just by changing my h = 0 point. say i'm holding a 3kg book over my head (two meters above my feet), but also say i'm on top of a 50 meter building. what's the potential energy of the book? is it mg2 (measuring from my feet) or is it mg52 (measuring from the ground)? "