Question

Placing the doorknob on the center will make it harder for you to swing it open or close, why?

Answer 1

This is because of torque as we know torque is force times radius. So for same amount of force torque will be high if force is acting at a greater distance from the axis of rotation,i.e higher value of r.

The case asked in the question is exactly same, if door knob is at center it will be more closer to axis about which door swings, hence less amount of torque generated for the same of force, hence it is always advisable to use the door knob as farther from axis of rotation as possible. Hope this will help!