that's what i meant! *sulks* no one understands me...
ok, rephrasing my previous post: you have object at x distance, you move it to z distance. so to calculate change in potential energy, you find the E at x, then the E at z, and then find the difference.
and my second sentence was just to correct your saying that r is the distance between the 2 points x and z. no, it's not: r is always the distance between the object and whatever the second object is tha it's all in reference to.
please tell me i make sense?