The rigid gate, $O A B$, of Fig. $\mathrm{P} 2.101$ is hinged at $O$ and rests against a rigid support at $B$. What minimum horizontal force, $P$, is required to hold the gate closed if its width is $3 \mathrm{m} ?$ Neglect the weight of the gate and friction in the hinge. The back of the gate is exposed to the atmosphere.