At a construction site, a 63.0 bucket of concrete hangs from a light (but strong) cable that passes over a light friction-free pulley and is connected to an 83.0 box on a horizontal roof (see the figure ). The cable pulls horizontally on the box, and a 45.0 bag of gravel rests on top of the box. The coefficients of friction between the box and roof are shown. The system is not moving.