Here on earth we assume an object on the ground has no potential energy. Not so!
Tell that to Indiana Jones standing on one of those tiles about to give way. If he has no potential energy, he would just float!
No - the brick always had a given amount of potential energy while on the surface of the earth.
In fact, it has equal potential energy anywhere in the system so long as Earth has gravity.
Remember on the ground, the force due to gravity pulling the brick toward the center of the earth is countered by the force of the ground preventing it from getting there.
In orbit, the force due to gravity has no countering force, thus it falls.
The instant before it strikes the ground, it's kinenic energy is equal to the potential energy it would have if it were just sitting there.
Now then, if the surface of the earth dissappeared at that instant, both would fall toward the center at the same acceleration. Hmm... but they wouldn't have the same kinetic energy once they reached it. drat.
