$解：(1)F_{浮}=G-F=30\ \text {N}-25\ \text {N}=5\ \text {N}$
$(2)V=\frac {F_浮}{ρ_水g}=\frac {5\ \text {N}}{1×10^{3}\ \text {kg/m}^3×10\ \text {N/kg}}=5×10^{-4}\ \text {m}^3$
$(3)m=\frac {G}{g}=\frac {30\ \text {N}}{10\ \text {N/kg}}=3\ \text {kg}$
$ρ=\frac {m }{V}=\frac {3\ \text {kg}}{5×10^{-4}\ \text {m}^3}=6×10^{3}\ \text {kg/m}^3$
