Answer
i) The torque $P$ creates causes the tipping.
ii) $P$ should be applied at the top left end of the crate.
$P_{min}=290N$
Work Step by Step
i) Because the tipping is in fact a rotation with the axis of ration at the obstruction, the torque $P$ creates causes the tipping.
ii) We have $\tau=Pl_P$. For $P$ to be minimum, $l_P$ needs to be maximum, which is when $P$ is applied at the top left end of the crate, as shown in figure b).
The axis of rotation is at the obstruction as shown in the figure. Before the crate tips, 3 forces create torques; net torque is zero.
$$\frac{Wl}{2}-Pl-\frac{Nl}{2}=0$$ $$Pl=\frac{l}{2}(W-N)$$ $$P=\frac{1}{2}(W-N)$$
When the crate tips completely off the ground, there is no more normal force, or $N=0$, so $$P=\frac{1}{2}W=290N$$
