A 15.0 m uniform ladder weighing 500 n rests against a frictionless wall. the stair forms an angle of 60.0 ° with the horizontal. (a) find the horizontal and vertical forces exerted by the ground on the base of the ladder when an 800 n firefighter is 4.00 m from the bottom. (b) if the ladder is about to slip when the firefighter is 9.00 m above, what is the coefficient of static friction between the ladder and the ground?