Science and Education Publishing International Journal of Partial Differential Equations and Applications 2376-9556 4 1 2016 7 20 Harmonic Oscillations and Resonances in 3-D Nonlinear Dynamical System 7 15 EN Usama H. Hegazy Department of Mathematics, Faculty of Science, Al-Azhar University, Gaza, Palestine Mousa A. ALshawish IJPDEA2016412 10.12691/ijpdea-4-1-2 2016 2 2 2016 6 14 2016 7 18 This paper is concerned with the three dimensional motion of a nonlinear dynamical system. The motion is described by nonlinear partial differential equation, which is converted by Galerkin method to three dimensional ordinary differential equations. The three dimensional differential equations, under the influence of external forces, are solved analytically and numerically by the multiple time scales perturbation technique and the Runge-Kutta fourth order method. Phase plane technique and frequency response equations are used to investigate the stability of the system and the effects of the parameters of the system, respectively.