I thought that when the ball hits the ground, at that exact moment the potential energy throughout the entire ball is 0. After seeing Simon's answer (thank you, by the way!) however, it seems that PE is 0 at the bottom of the ball.
This is a question that me and a friend have been wondering. He says that when the ball hits the ground and is at rest for a very brief moment, potential energy will be 0 in the center of the ball, while I thought that it doesn't matter as the entire ball's potential energy is at 0. Which answer...