The characteristicsof a vector are the magnitude, the orientation, and the sense. The magnitudeof a vector is speci®ed by a positive number and a unit having appropriate dimensions. No unit is stated if the dimensions are those of a pure number. The orientationof a vector is speci®ed by the relationship between the vector and given reference lines and=or planes. Thesenseof a vector is speci®ed by the order of two points on a line parallel to the vector. Orientation and sense together determine the directionof a vector. Theline of action of a vector is a hypothetical in®nite straight line collinear with the vector. Vectors are denoted by boldface letters, for.
