漂浮,浮力F=重力G
F=p水*V排*g=p水*(1-2/5)V木*g=0.6p水*V木*g
G=mg=p木*V木*g
0.6p水*V木*g=p木*V木*g
p木=0.6p水=0.6g/cm^3=600kg/立方米