I need help on part b, I calculated a...PLEASE!?

Question

A 15.0 m, 508 N uniform ladder rests against a frictionless wall, making an angle of 60.0° with the horizontal.
(a) Find the horizontal and vertical forces exerted on the base of the ladder by the Earth when an 814 N fire fighter is 4.00 m from the bottom. horizontal N 271.97N
vertical N 1322N
(b) If the ladder is just on the verge of slipping when the fire fighter is 9.00 m up, what is the coefficient of static friction between ladder and ground?

CAn someone give me a clue how to do part b? I calculated part a and I needed help on b! Thanks

Answer 1

If you can do part a, then you can determine the horizontal and vertical forces on the base of the ladder when the fire fighter is 9m up. Let the horizontal force you calculate be H and the vertical force V

The vertical force V that you calculated is the normal force N. The frictional force is at most Ks N, where Ks is the coefficient of static friction. Since the base of the ladder isn't moving, the net horizontal force is 0. You've already computed the magnitude that is trying to move it away from the wall, H. This is being resisted by the force of friction. The two must be equal. So at the point of slipping:

H = Ks N

Since you know H and N, ...