球体体积V,空心体积V'
p(V-V‘)g=p水gV/3
p(V-V')g+2p水gV'/g=p水gV
2V'/5=5*10^5
由此可求金属密度p,V’,V
N+F浮=G
则N=p(V-V')g+p水gV'-p水gV