Door Hinge:

Door hinge connects the door with the wall. Door hinge is a simple example of Newton’s third Law of physics, as the force acting on the wall by the door is equal to the force acting on the door by the wall and acting on opposite direction. In this particular case, keeping in mind the general principle of mechanics, door and the hinge are in equilibrium. Perpendicular component of the force acting on the door knob produces a moment about the door hinge. It is easiest to open the door by pushing the door knob as the moment is larger the farther away it is from the hinge. Pushing at an oblique angle to the door is less effective and the door will not open if one pushes towards the hinge.

(Picture taken by Ahsan Iqbal, Date: 03/02/09, Topic: Door Hinge)