For our first physical realization of a system exhibiting wave
behavior, we use the string system in Figure 1. The
string stretches between two vertical walls held at a distance L
apart, one wall at x=0 and the other at x=L. The wall at x=0
holds the string in place at y=0, where the string attaches to a
pin. The wall at x=L has a small hole at y=0 through which the
string passes. This ensures that the string at this end also is at
position y=0 while allowing us to apply a force F to keep the
string under constant tension 
. Otherwise, the string is free
to move. The figure shows an example where the string has been
``plucked'' so that it now supports a wave as sketched in the figure.
The total mass of the length of string between the walls is M and,
thus, the mass per unit length of the string is 
.
Figure 1: String system illustrating simple wave behavior