I just have a kind of conceptual question. I am doing a problem. It's related to a ball rolling from a inclined plane without slipping. The answer uses the Conservation of Mechanical Energy to attack that problem. But isn't true that Conservation of Mechanical Energy can only be used when there isn't any nonconservative force? The force of friction between the plane and the ball is a nonconservative force, isn't it? So why can it use the Conservation of Mechanical Energy to do that problem? Overall, my problem is simply. Why is Mechanical Energy conserved in "rolling without slipping on inclined plane", with the nonconservative force of friction? It's my first post in this forum. thank you.