Dùng mặt phẳng nghiêng dài 8m để kéo vật có khối lượng 49,2kg lên cao 2m. a) tính công kéo vật có khối lượng theo mặt phẳng nghiêng khi không có ma sát b) tính lực kéo vật lên theo mặt phẳng khi không có ma sát c) thực tế có ma sát lực kéo vật là F=150N.Tính hiệu suất của mặt phẳng nghiêng

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