is a toothed bar or rod that can be thought of as a sector gear with an
infinitely large radius of curvature. Torque can be converted to linear
force by meshing a rack with a pinion: the pinion turns; the rack moves in a
straight line. Such a mechanism is used in automobiles to convert the
rotation of the steering wheel into the left-to-right motion of the tie
rod(s). Racks also feature in the theory of gear geometry, where, for
instance, the tooth shape of an interchangeable set of gears may be
specified for the rack (infinite radius), and the tooth shapes for gears of
particular actual radii then derived from that.