The second step in understanding a new phenomena is to identify the
basic quantities which we hope to describe and understand. Under
appropriate conditions (demonstrated in lecture), vibrations exist
where each little segment of the string in Figure 1
moves up and down periodically. Wave motion often involves such
periodic motion in time, something whose characterization we reviewed
in the previous set of notes. Note that also that the shape of the
string shown in the Figure 1 repeats every time we
move a distance 
down the
string. Thus, waves exhibit periodic behavior not only in time but
also in space.