Hey there. I have a problem about rolling ball and it's from my homework. 1. The problem statement, all variables and given/known data A ball rolls down the surface of a rough fixed sphere of radius r, starting rest at the top. What's its speed when the line between the two centres makes an angle alpha with the upward vertical? Also find the normal component of the reaction force on the ball at this time, and show that the ball leaves the surface when cos(alpha)=10/17. 2. Relevant equations I=2/5ma^2 3. The attempt at a solution KE=mg(r+a)(1-cos(alpha))=7/10mu^2 so u=(10/7g(r+a)(1-cos(alpha))) ^1/2 this should be the answer to the first part of the question. I don't quite have a clue to do the second part. How to find the normal component of the force? It has been annoying me for days and thank anyone who helps me with it!