Bài 7: Người ta kê một tấm ván để kéo một cái hòm khối lượng 60kg lên một chiếc xe tải. Sàn xe tải cao 0,8m so với mặt đường, tấm ván dài 2,5m, lực kéo bằng 300N a) Tính lực ma sát giữa đáy hòm và mặt ván. b) Tính hiệu suất của MPN này Bài 8: Một máy kéo có công suất 40kW a) Con số đó cho ta biết điều gì? b) Tính công của máy sinh ra trong 4 giờ. c) Tính lực kéo của máy Biết trong thời gian đó xe chuyển động đều và quãng đường xe đi được là 200km. Bài 9: Một máy hoạt động với công suất 1,2kW để nâng một vật lên cao 10m. Hỏi thời gian để nâng vật lên là bao nhiêu? Biết vật có khối lượng 90kg, hiệu suất của máy là 75%

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