2.Machines for Lifting

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