Example 2 :

A vertical rectangular gate of 4m x 2m is hinged at a point 0.25 m below the centre of gravity of the gate. If the total depth of water is 7 m what horizontal force must be applied at the bottom to keep the gate closed?

Given data: Area of the gate = 4x2 = 8 m ²

                   Depth of the water = 7 m

                  Hydrostatic force on the gate

Taking moments about the hinge we get,

Answer: 18.8 kN.