Problem $3\mathrm{.}4$ A circular opening, $3m$ diameter, in a vertical side of a tank is closed by a disc of

$3m$ diameter which can rotate about a horizontal diameter. Calculate :

$($i$)$ the force on the disc, and

$($ii$)$ the torque required to maintain the disc in equilibrium in the vertical position when the head

of water above the horizontal diameter is $4m.$

$($A$.$M$.$I.E$.,$ Winter, $1977)$