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.
Notation
Meaning
a or a
A vector named a; bold type or underlining distinguishes it from a scalar.
AB
The vector directed from A to B.
(xy)
A column vector: horizontal component first, vertical component second.
xi+yj
The same vector written using perpendicular unit vectors i and j.
Read the components
i points along the positive x-axis and j along the positive y-axis. Each has magnitude 1.
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 A to B means moving 3 units right and 4 down:
AB=(3−4)=3i−4j.
For comparison, (−25)=−2i+5j represents 2 units left and 5 up.
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 starts at A, not at B.