A hollow, spherical shell with mass 1.95 rolls without slipping down a slope angled at 30.0. Find the minimum coefficient of friction needed to prevent slipping. I have already calculated the acceleration of the sphere and the magnitude of the friction force on the sphere in previous parts. I used torque and newton's laws to find the acceleration and friction. They are: a = 2.94 m/s^2 f = 3.82 N I used the following equation for the sum of forces in the x-direction, which I oriented along the slope of the incline, with the positive direction down the incline. I didn't think I needed to use torque here, because I got the same answer that way as well. (sum)Fx = mgsin(theta) - f = ma = mgsin(theta) - (mu)mgsin(theta) = ma mu = [a - gsin(theta)] / [-gsin(theta)] mu = (-1.96 m/s^2) / (-4.9 m/s^2) = 0.4 This coefficient of friction isn't right, but I'm sure I included all of the forces. Anyone know where I messed up? Thanks.