1. The problem statement, all variables and given/known data Consider a small frictionless puck perched at the top of a fixed sphere of radius R. If the puck is given a tiny nudge so that it begins to slide down, through what vertical height will it descend before it leaves the surface of the sphere? 2. Relevant equations 3. The attempt at a solution I recognize that when the normal force on the puck exerted by the sphere is zero, the puck is no longer on the sphere. Therefore, I seek to express the normal force as a function of height, or of the angle measured down from the vertical, from which the height is easily calculated. I can't find a way to write the normal force. What would the normal force be, at a point? Looking at a solution, someone wrote: N = mgcosø + (mv^2)/R where ø is the angle measured down from the vertical, so that it could also be written N = (mgh + mv^2)/R Where does this come from? I don't see the truth in this expression of the normal force.