1. The problem statement, all variables and given/known data A hollow spherical shell with mass 2.50kg rolls without slipping down a slope that makes an angle of 33.0 with the horizontal. Find the minimum coefficient of friction μ needed to prevent the spherical shell from slipping as it rolls down the slope. 2. Relevant equations 3. The attempt at a solution I already found force of friction = 5.34N. It seems that I can get the minimum by setting force of friction = μN So: 5.34 = μmgcos(Θ) What I don't understand is why this is the minimum of coefficient of friction. How can one assume that this value is the minimum coefficient of friction to prevent slipping?