设木块密度ρ,体积为V,水密度为ρ',木块浮在水面,浮力=ρ'*(0.8V)*g=重力=ρVg==>ρ=0.8ρ'=800kg/m^3;
对该物体,密度为ρ,全部浸入水中,受到弹簧拉力0.8ρVg,重力ρVg,浮力=ρ'Vg：浮力+拉力=重力==>
ρ'Vg+0.8ρVg=ρVg==>ρ=5ρ'=5000kg/m^3