F浮=ρgV=1.29*9.8*4.5*0.001=0.0569NG=mg=0.003*9.8=0.0294NF拉=F浮-G=0.0569-0.0294=0.0275N
4.5*1.29-3=2.805
氢气密度多少阿?具体做法拉力=浮力-重力