Weight required to be placed on the top of the wood is,
          Â
W = Upthrust force - Weight of the wood
              Â
= (density of water- density of wood) V g
               Â
= (1000 kg/m3 - 750 kg/m3)(0.4
m)3(9.8 m/s2)
                Â
= 156.8 N
Mass required to be placed on the top of the wood is,
            Â
M = W / g
               Â
= 156.8 N / 9.8 m/s2
               Â
= 16 kg
