5. Discussion and conclusions
Application of Newtons law to the motion of the simple pendulum with nonlinear damping terms and incorporating
several drivings leads to the equations of motion of what we call a generalized perturbed pendulum. This model is a
paradigm for the study of many properties of continuous dynamical systems, with many applications to physical and
technological problems.
Fig. 9. Semilogarithmic plot of the Melnikov ratio versus the forcing frequency x for the values x0 ¼ 0:5 (––), x0 ¼ 1 (– – –) and
x0 ¼ 1:5 (- - -). This example represents a simplified version of the case when the three forcing terms are acting at the same time.
J.L. Trueba et al. /Chaos, Solitons and Fractals 15 (2003) 911–924 921
We apply Melnikov method, which has proved useful in many practical cases to ascertain the chaotic responses of
certain dynamical systems, to the generalized harmonically perturbed pendulum, resulting in general expressions that
comprise particular cases. Among all the examples analyzed, an interesting particular situation appears when the
supporting point of a linearly damped pendulum moves harmonically in the horizontal direction and the bob is subjected
to a harmonic forcing, because there is a set of forcing parameters that removes the effect of the driving from the
Melnikov function.
One of the strategies we have pursued here is analyzing the Melnikov ratio between the critical forcing amplitude
and the damping coefficient for some specific examples. Its dependence with the natural and forcing frequencies has
been determined for each case.
Finally, we would like to stress that even though most of the particular cases of this generalized perturbed pendulum
have been studied separately by different authors, here we have attempted to offer a general scheme comprising all them
and also showing general formulae that can be of further use, and that which might be extended to attack other
problems in nonlinear dynamics.