解：$(1)F_{浮}=G-F=3.4\ \text {N}-2.9\ \text {N}=0.5\ \text {N}$
$(2)V=\frac {F_浮}{ρ_水g}=\frac {0.5\ \text {N}}{1×10^{3}\ \text {kg/m}^{3}×10\ \text {N/kg}}=5×10^{-5}\ \text m^{3}$
$(3)m=\frac {G}{g}=\frac {3.4\ \text {N}}{10\ \text {N/kg}}=0.34\ \text {kg}$
$ρ=\frac {m }{V}=\frac {0.34\ \text {kg}}{5×10^{-5}\ \text m^{3}}=6.8×10^{3}\ \text {kg/m}^{3} $
