để đưa một vật khối lượng m = 200kg lên độ cao h =10m người ta dùng một trong hai cách sau a, dùng hệ thống gồm một ròng rọc động biết hao phí để nâng ròng rọc động bằng hao phí tổng cộng do ma sát b, dùng mặt phẳng nghiêng l = 12 . Lực kéo vật lúc này là F2 = 1900N. Tính lực ma sát giữa vật và mặt phằng nghiêng , hiệu suất của cơ hệ này

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