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一个无盖的长方体木箱,长0.8m,宽0.5m,高0.4m,它占...

(1) p= F S = 500N 0.4 m 2 =1250Pa .答:木箱对地面的压强是1250Pa.(2)W 有用 =Gh=500N×0.5m=250J.W 总 =Fs=80N×4m=320J. η= W 有用 W 总 = 250J 320J =78% .答:有用功是250J,斜面的机械效率约是78%.

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