The Columbia Encyclopedia, Sixth Edition. Columbia Encyclopedia World Factbook Columbia Gazetteer American Heritage Coll.
Many physical quantities are vectors, e.g., force, velocity, and momentum.
Thus, in specifying a force, one must state not only how large it is but also in what direction it acts. The simplest representation of a vector is as an arrow connecting two points. In order to compare vectors and to operate on them mathematically, however, it is necessary to have some reference system that determines scale and direction. Cartesian coordinates are often used for this purpose. In the plane, two axes and unit lengths along each axis serve to determine magnitude and direction throughout the plane.