（1）大桥用钢所受的重力：G=mg=4.6×107kg×10N/kg=4.6×108N；（2）∵ρ= m v ∴大桥用钢的体积：v= m ρ = 4.6×107kg 8.9×103kg/m3 =5168.54m3．答：（1）大桥用钢所受的重力为4.6×108N；（2）大桥用钢的体积是5168.54m3．
