V排=40cm3=4x10-5m3F浮=PgV排=1X103Kg/m3x10N/kgx4x10-5m3=0.4N
∵F浮力=ρ水gV排→V排=40ml,g=10N/kg,ρ水=1g/cm^3∴F浮力=1g/cm^3×10N/kg×40ml=40g×10N/1000g=0.4N