As you can see in Figure 3-1, a bar is fixed at one and is set into vibration by tapping it from the bottom. Imagine that a strobe light is used to take a series of snapshots of the bar as it vibrates up and down.
At each snapshot the instantaneous displacement of the bar is measured. Instantaneous displacement is the distance between the rest position of the bar (defined as zero displacement) and its position at any particular instant in time.
Positive numbers signify displacements that are above the rest position, while negative numbers signify displacements that are below the rest position. The vibratory pattern that is traced out when the sequence of displacements is graphed is called a sinusoid.
