Opening Doors

The mechanics of the seemingly simple task of turning a doorknob involves a variety of forces that the hand exerts on the knob and through it to the door. The opposing forces produced by the thumb and fingers together form what engineers call a couple, which consists of a pair of equal and opposite forces that collectively produce a turning motion rather than a translational one. Thus, when the spindle of a small toy top is twisted, a couple is applied to it, and it tends to spin in place rather than fly across the table. In the US, inventors and manufacturers began to think seriously about levers as alternatives to knobs in the years leading up to the Americans with Disabilities Act, which was signed into law in 1990. Like most legislation, the act was preceded by an increasing sensitivity to the problems it would address.