Fluid Statics: Calculate Moment on Rectangular Gate Hinged on Upper Edge

[physocs1]

Homework Statement

The rectangular gate shown in section is 3 m long (perpendicular to the paper) and is hinged about its upper edge B. The gate divides a channel leading to a fresh-water lake on the left and a salt-water tidal basin on the right. Calculate the moment, M, on the shaft of the gate at H required to prevent the gate from opening when the salt-water level drops to h = 1 m. Take the density of salt water 1050 kg/m3

Homework Equations

1. Force=rho*g*v
2. F=gamma*h*a
3.R= rho*g*h*a
Where gamma = specific weight of liquid
Where R=hydrostatic pressure

The Attempt at a Solution

Calculate Both forces acting on rectangle gate. Then solve for the moment
Using sum of moments

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I am confused on which equation to use. I want to use 3. since its a flat surface but I confused whether to use gravity in the equation?


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[physocs1]

On another note: In the attached png. I am confused on they calculated "3/2" for the height
I am thinking if its the centroid?