At a construction site, a 65.O-kg bucket of concrete hangs from a light (but strong) cable that passes over a light friction-free pulley and is connected to an SO.O-kg box on a horizontal roof (Fig. 7.29). The cable pulls horizontally on the box, and a 50.O-kg bag of gravel rests on top of the box. The coefficients of friction between the box and roof are shown. (a) Find the friction force on the bag of gravel and on the box. (b) Suddenly a worker picks up the bag of gravel. Use energy conservation to find the speed of the bucket after it has descended 2.00 m, from rest. (You can check your answer by solving this problem using Newton's laws.)

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Box -0.700 " Q400 Concrete