A gate closing an opening is triangular in cross section and 1 m long. It is hinged on the top and freely supported at one of the bottom ends as shown in the figure. If the gate weighs 25 kN/m ³ find the height of the water that will automatically open the gate.

          Let, the height of the water h above the bottom of the Gate

          Weight of the gate (acting downward)
                 acting at 1/3 m from B .
          Force on the vertical surface acting horizontal direction

                               acting at h/3 m above B.
          Upward pressure on the horizontal surface

                                       acting 1/2 m from B.

          Equate the sum of moments about B to zero.

          Answer: height of the water h = 0.438m.