A force is a vector quantity, and, as a result, it has both magnitude and direction. These properties allow a geologist to manipulate forces following the principals of vector algebra. For example, consider some force, F, acting parallel to the y-axis on line segement AC as illustrated in Figure 2.
Now ask the question, what are the components of this force acting normal and parallel to line segment AC? In order to answer this question, resolve F into its two vector components Fn and Fs. Fn is the component of F acting normal to AC and Fs is the component of F acting parallel to AC. Fn is referred to as the normal force whereas Fs is termed the shearing force. Trigonometric relationships displayed in Figure 2 indicate that:
and:
where q is the angle between Fn and F.