???????????????
?????????????????????λ????? kg ???????????????
W ???????? kg ?? = F ??????? mm2 ???? L ??????? m ??????????? g/cm3 ???? 1/1000
????????? 7.85g/cm3
??壨kg/m2??W= 7.85 ??d d= ??
??? 4mm ???壬???m2 ???????m2 ????=7.85 ??4=31.4kg