A hoist for lifting lightweight building materials is shown. The pulley at B is frictionless, and the pulley at C is driven by an electric motor rotating counterclockwise at a speed high enough to always cause slip between the cable and pulley C. The cable makes contact with one-fourth of the surface of pulley C, and the coefficients of static and kinetic friction are 0.25 and 0.2, respectively. If the cable has negligible weight and the building materials and platform at D have a combined weight of 100 lb, determine the force the worker must apply to the cable to: D A 75 (a) Raise the building materials at a uniform speed. A (b) Lower the building materials at a uniform speed. (c) Hold the building materials in a fixed, suspended position