Representation of Vector

A vector is drawn as a directed line segment: if $A$ and $B$ are two points and the segment $AB$ carries

an arrowhead at $B$, the resulting directed segment represents the vector $\overrightarrow{AB}$ (read "AB

bar" or "vector AB"), with $A$ called the initial point and $B$ the terminal point.

Reading the same

segment the other way gives the vector $\overrightarrow{BA}$, which has the same length but the opposite

sense. The full (unbounded) straight line through $A$ and $B$ is called the vector's line of support or

line of action. When the initial and terminal points of a vector are not specified, it is written using a

single lower-case letter with a bar or arrow, e.g. $\bar a, \bar b, \bar c$.

Example (identifying vectors from a diagram). Given a picture of several arrows $\bar a,\bar b,\bar c,\bar d,\bar e$ of assorted lengths and directions, one can read off directly from the picture: which are equal in magnitude (same length, direction irrelevant), which are parallel (same or exactly opposite direction),

which point in the same direction, which pairs are equal vectors (same length AND same direction), and …