$($b$)$ A rectangular sluice door is hinged at the top at

$\text{'}A\text{'}.$ The door is $1200$ mm wide, $900$ mm long

and has a mass attached which moves the centre

of gravity of the door to $\text{'}G\text{'}.$ The total weight of

the door with the attached mass is $9810$ N $.$

Find the minimum force required at $B$ to keep

the door closed if the height of water is $1\mathrm{.}5$ m $.$