The problem is a boy is seated at the top of a hemispherical mound of ice so theres no friction. The radius of the mound is R. He is given a very small push so at the top there is basically only potential energy with a height of R. After he starts sliding down i need to show that the point where he leaves the ice is at a height of 2R/3. I know this point would be where the normal force is equal to zero. Since the normal force is zero i set the centrepital force equal to the weight at that given moment but im not sure if thats right. Any help is greatly appreciated.