1. The problem statement, all variables and given/known data A hollow cone is put upside-down with its symmetry axis vertical. The surface of it makes an angle of theta with the vertical direction as shown in the figure . A small puck of mass m slides without friction on the inner side of this cone and remains within a horizontal plane as it rotates about the axis of the cone. EDIT: The question: What is the normal force exerted on the puck by the inner surface of the cone in terms of m, g, and theta? 2. Relevant equations 3. The attempt at a solution 1) Resolve X and Y coordinate so that the surface of the inside of the cone is x and y is perpendicular to that surface. 2) Resolve force of gravity into x and y components. 3) Fy = Fn - mgsin(theta) = 0 Fn = mgsin(theta) (WRONG) The actual solution 1) Set X and Y coordinate normally 2) Resolve the NORMAL force 3) Fy = Nsin(theta) = mg N = mg/sin(theta) So I'm always used to resolving gravity into components when doing force problems, and so I did that. However, to get the correct solution in this problem you must resolve the normal force into components. I'm not sure why one way is wrong and the other way is correct.