先由质量算出体积 v = m/p =178kg/ 8.9克/厘米3 =20000厘米3=20000000mm3
再由体积算出长度 铜丝直径1mm 横截面积=兀r2=0.785mm2
20000000/0.785mm2=25477707mm约等于25km .
好长啊...