Answer
$F = 3.96\times 10^4~N$
Work Step by Step
We can find the total pressure at the bottom of the cube:
$P = P_{atm}+P_g$
$P = (1.00~atm)+\rho g h$
$P = (1.01\times 10^5~N/m^2)+(1030~kg/m^3)(9.8~m/s^2)(0.900~m)$
$P = 1.10\times 10^5~N/m^2$
We can find the magnitude of the force pushing upward on the bottom of the cube:
$F = P~A$
$F = (1.10\times 10^5~N/m^2)(0.600~m)^2$
$F = 3.96\times 10^4~N$
