= Solution
Parallel elements have the same <strain>, while their <stresses> add. The spring supplies $G_0\epsilon$ and the viscous element supplies $\eta_0\dot\epsilon$, so the <Kelvin-Voigt model> has <constitutive equation>
$$
\boxed{\sigma(t)=G_0\epsilon(t)+\eta_0\dot\epsilon(t).}
$$
For the oscillatory deformation this gives
$$
\sigma(t)=(G_0+i\omega\eta_0)\epsilon_0e^{i\omega t},\qquad
\boxed{G^*(\omega)=G_0+i\omega\eta_0.}
$$
Physical <stress> and <strain> are the real parts of these expressions. Thus the <storage modulus> is $G_0$, independent of frequency, and the <loss modulus> is $\omega\eta_0$. The equation is the quasistatic material law; an inertial equation for an attached mass would require additional mechanical data.
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