The elastic and inelastic responses of the force-excited single-degree-of-freedom (SDOF) systems are investigated from the gross characteristics of forcing functions such as the frequency content, the integrals and the amplitudes. The studies are carried out initially with simple version of force-excitation and then, using progressively complicated version of forcing functions having more than one frequency. The forces are chosen in such a way that they resemble the simple version of wave-induced forces, thus, paying a special attention to compliant systems. The responses due to actual wave-induced forces are also studied considering the surface record from the Gulf of Mexico during Camille hurricane (August, 1969). The responses are displayed in tripartite response spectra and explained from the characteristics of the forces. The effect of the frequency content in the forces and their integrals are addressed while interpreting the results. The studies are also made based on the parameters such as damping of the systems and the periodicity of forcing functions. An attempt is made to approximate the elastic responses from the characteristics of the integrals of forcing functions in low and low-medium frequency region. The inelastic responses are studied based on the gross characteristics of forcing function and they are compared with the elastic responses. Low-frequency systems of high ductility and low ductility are given a special attention due to current interest in compliant systems.