Sentence:  A doorknob of whatever roundish shape is effectively a continuum of levers, with the axis of the latching mechanism&mdash;known as the spindle&mdash;being the fulcrum about which the turning takes place. Applying a <WSD>force</WSD> tangential to the knob is essentially equivalent to applying one perpendicular to a radial line defining the lever. 
Meaning of the word marked with <WSD> tags:  Something or anything that has the power to produce a physical effect upon something else, such as causing it to move or change shape.