4. A long, vertical wall separates seawater from freshwater. If the seawater stands at a depth of 7m, what depth of freshwater is required to give a zero resultant force on the wall? When the resultant force is zero will the moment due to the fluid forces be zero? (Seawater: y=10.1KN/m³; freshwater: y=9.80 KN/m³).

Fig: 1