$解：?(1)F_浮=G_排=0.5\ \mathrm {N}?$
$? (2)V=\frac {F_浮}{ρg}=\frac {0.5\ \mathrm {N}}{1.0×10^3\ \mathrm {kg/m}^3×10\ \mathrm {N/kg}}=5×10^{-5 }\mathrm {m^3}?$
$? (3)ρ_液=\frac {F'_浮}{Vg}=\frac {mg}{Vg}=\frac mV=\frac {55×10^{-3}\ \mathrm {kg}}{5×10^{-5}\ \mathrm {m^3}}=1.1×10^3\ \mathrm {kg/m}^3?$
