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This work focuses on dynamic modelling of a simple planetary gear. The dynamic model comprises of a carrier, sun gear and two planets for simplicity. Each component has three degrees of freedom in the radial, tangential and rotational direction (see Fig.1); this implies that the overall dynamic model is a nine degree of freedom system, which is subsequently written in state space form. The free and forced dynamic responses of the planetary gear were determined, first using a fixed reference frame and then a reference frame rotating with the carrier. The natural frequencies and mode shapes obtained in the fixed reference frame are identical with the results using the rotating reference frame if the carrier speed is zero. The relationship between the natural frequencies and the carrier speed over a broad speed range of 0 to 10000 rpm was also investigated in the rotating reference frame. Significant changes in the natural frequencies due to the Coriolis effect induced by the carrier speed can be observed. Force excitation, which depends on the mesh stiffness and transmission error at the pressure line, was applied to the system with viscous damping being present at the supports of the sun and two planets considering both the two reference frames. This research will serve as a source of information for the engineers in interpreting results obtained from either a fixed or rotating reference frame when characterising the dynamic behaviour of planetary gears.