A rectangular gate shown below $($Fig$.7\mathrm{.}7)$ separates water in the reservoir from the tail water

in a tunnel. The gate is $3\mathrm{.}0m$ wide and is hinged at $H$ as shown in the diagram. It is observed

that as the water rises in the reservoir, pressure will build up on the gate until the gate is forced

to open. This happens when the level of water in reservoir, $(h),$ reaches a level of $2\mathrm{.}5m$ above

the hinge. Thus, the gate is designed so that it just opens when the water level $(h)$ in the reservoir

rises to $2\mathrm{.}5m$ above the hinge as mentioned. If the thickness of the gate is assumed to be

negligible, determine the mass of this gate.