A worker pushes on a mop with a force F. The handle is at an angle θ with the vertical and μs and μk are the coefficients of static and kinetic friction between the head of the mop and the floor. Ignore the mass of the handle and assume that all the mop's mass m is in its head. a) If the mop head moves along the floor with a constant velocity, then what is F? b) Show that if θ is less than a certain value θ0, then F is unable to move the mop head. Find θ0 i got part a) to be F = μk(mg) / (sinθ - μkcosθ). but i can't get part b). all i have is Fn(μs) = Fsinθ0 where Fn is the normal force. the book's answer is θ0 = tan-1(μs). how did they get that?