In Chapter 4 we discussed the normal modes of a coupled oscillator. The striking
characteristic of a normal mode is that all the masses move in SHM at the same
frequency: indeed this defined the normal modes. We also saw that these normal
modes are completely independent of each other and the general motion of the
system is a superposition of the normal modes. All of these properties are shared
by standing waves on a vibrating string; all the particles of the string perform
SHM with the same frequency. Indeed the standing waves are the normal modes
of the vibrating string and from now on we shall generally refer to them as normal
modes. So far we have only considered the case in which a single normal mode of
the string is excited. In Section 6.4.2 we shall deal with the case in which several
normal modes are excited simultaneously. We shall discuss their superposition and
independence and again we will see much similarity with our discussion of normal
modes in Section 4.3. The methods and results that we shall demonstrate for a
vibrating string admit generalisation to a huge range of physics – for example to
quantum mechanics – and are therefore of great importance. We shall begin by
describing the superposition principle.