We saw in Chapter 3 that the amplitude of oscillation of a harmonic oscillator becomes very large when a periodic driving force is applied at its natural frequency of oscillation. At other driving frequencies the amplitude is relatively small. For the case of two oscillators coupled together we may expect similar behaviour. Now, however, there are two natural frequencies corresponding to the two nor-mal frequencies. Thus we may expect that the system will exhibit large amplitude oscillations when the driving frequency is close to either of these two normal fre-quencies. This is indeed the case. We can explore forced oscillations by considering the arrangement of two masses connected by springs as shown in Figure 4.15. This is similar to the arrangement shown in Figure 4.10 but now the end s of one of the outer springs is moved harmonically as ξ = a cosωt. The displacements ξ,xa
and xb of the masses from equilibrium are shown in Figure 4.15 at some instant