A square flap with a weight G covers the outlet of a water tank. The flap has a pivot axis O. Determine the vertical force F needed to lift the flap.
Data:
\begin{aligned} &h_1 = 3.5 m\\ &h_2 = 2.5 m\\ &G = 200 N\\ &\alpha = 30^{\circ}\\ \end{aligned}