We use vectors to describe any physical quantity which has both a magnitude and a direction. Common examples of such quantities are velocity, acceleration, force, the electric field, and the magnetic field. In the three-dimensional world in which we live, such vectors are specified by giving three components. For instance, we might write the velocity of a moving object in either of the forms
or
We will usually write vectors using unit vectors, as in the second form above.
The components 
and 
tell how rapidly the object is
making progress along each of the coordinate axes.
The magnitude of the velocity is given by the familiar Pythagorean formula:
