2.Machines for Lifting

Thestrongestathletescanliftcloseto455kgjustofftheground,butfewordinarypeoplecanliftmorethanabout68kg.Earlymansoondevelopedmachinesforliftinglargestonesandtreetrunks.Asimpledeviceisasinglepulleywheelarrangedasahoist.Butaropearoundonepulleymerelychangesthed ......     (本文共 5981 字 )     [阅读本文]>>




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