Vectors in Two Dimensions

01Vectors in Two Dimensions

Vector Notation

This section covers directed line segments, column vectors and notation using the perpendicular unit vectors i and j.

Magnitude and direction

A vector has both magnitude and direction. A directed line segment represents it: the line's length indicates the magnitude, and its arrow points in the direction of the vector.

NotationMeaning
a\mathbf a or a‾\underline aA vector named aa; bold type or underlining distinguishes it from a scalar.
AB→\overrightarrow{AB}The vector directed from AA to BB.
(xy)\begin{pmatrix}x\\y\end{pmatrix}A column vector: horizontal component first, vertical component second.
xi+yjx\mathbf i+y\mathbf jThe same vector written using perpendicular unit vectors i\mathbf i and j\mathbf j.

Read the components

i\mathbf i points along the positive xx-axis and j\mathbf j along the positive yy-axis. Each has magnitude 11.

A positive horizontal component means right; a negative one means left. A positive vertical component means up; a negative one means down.

In the diagram, going from AA to BB means moving 33 units right and 44 down:

AB→=(3−4)=3i−4j.\overrightarrow{AB}=\begin{pmatrix}3\\-4\end{pmatrix}=3\mathbf i-4\mathbf j.

For comparison, (−25)=−2i+5j\begin{pmatrix}-2\\5\end{pmatrix}=-2\mathbf i+5\mathbf j represents 22 units left and 55 up.

AB3right4down¡!AB
The component signs come from the direction of travel, not the arrow's length.

Exam Tip: In handwritten work, underline a single-letter vector or use an arrow over two endpoint letters. Read those letters in order: AB→\overrightarrow{AB} starts at AA, not at BB.

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