解：$(1)G=mg=900\ \mathrm {kg}×10\ \mathrm {N/kg}=9000\ \mathrm {N}$
$(2)p=\frac {F}{S}=\frac {9000\ \mathrm {N}}{4×0.025\ \mathrm {m^2}}=9×10^4\ \mathrm {Pa}$
$(3)F'=p'S=2×10^5\ \mathrm {Pa}×4×0.025\ \mathrm {m^2}=2×10^4\ \mathrm {N}$
$m_{总}'=\frac {G'}{g}=\frac {F'}g=\frac {2×10^4\ \mathrm {N}}{10\ \mathrm {N/kg}}=2000\ \mathrm {kg}$
$m_{总}=m_{总}'-m=2000\ \mathrm {kg}-900\ \mathrm {kg}=1100\ \mathrm {kg}$
