Verified answer

You mean a doorstop? You wrote doorstep.

Doorstop:

http://moblog.net/media/m/i/c/microhappy/tony-danz...

The reason it is more effective further from the hinge:

A doorstop is ALWAYS demanded to supply enough torque about the hinge to fully oppose the torque supplied by the automatic closing spring.

If the doorstop somehow fails to supply this torque, it will skid and let the door close.

In order for the doorstop to apply the torque to the door, three physics concepts can be adjusted to make it do so:

1. the force it exerts on the door

2. the radial coordinate of that force (i.e. how far it is placed from the hinge)

3. the perpendicularity of the force to that radial coordinate (i.e. the sine of the angle between vectors 1 and 2)

The total torque is the cross product of vectors 1 and 2.

tau = r cross F

Or in otherwords, as a magnitude, it is a conventional product of all three terms as scalars multiplied by each other:

tau = r*F*sin(theta)

We control theta by wedging the doorstop perpendicular to the door. It would be foolish to wedge it parallel to the door.

We control r by placing the doorstop as far from the hinge as we can. If you placed it directly under the hinge, it'd do no good.

The wedge itself controls F by a feedback system. When the door closes too much, the wedge compresses and the floor can exert greater traction force on the wedge/door. When the door doesn't completely contact the wedge, the spring on the door pushes it more in to the wedge.

In order to minimize mechanical failure to the doorstop wedge and the door, you want to MINIMIZE the stress of contact, and you do so by reducing the demand for traction force.

Moral of the story, since it "costs you nothing" to place the doorstop far from the hinge, you MIGHT AS WELL do so, because it doesn't demand as much force from the wedge.