How do you find the minimum coefficient of friction needed to prevent slipping for part a and part b. A hollow spherical shell of mass 4.00 kg rolls without slipping down a 38.0° slope. (a) Find the acceleration. 3.62 m/s2 Find the friction force. 9.64 N Find the minimum coefficient of friction needed to prevent slipping. (b) How would your answers to part (a) change if the mass were doubled to 8.00 kg? Acceleration 3.62 m/s2 Friction force 19.31 N Coefficient of friction for part a I already tried setting mgsinθ-μmgcosθ = 0 and got 0.781 but it was wrong.