Why a force is applied at right angles to the heavy door at the outer edge while closing or opening it?

Explanation

Torque `tau=F r sin theta`
For a force applied at right angle to the outer edge of the door i.e. `theta=90^(@) :' sin 90^(@)=1`
`:. tau=Fr`
`:.` The values of `sin theta` and r are maximum.
So, torque produced is maximum. Hence force is applied at right angles to the heavy door at the outer edge.