Throughout our analysis, we have assumed that both the drop and bath deformations are dissipated between impacts, an assumption that breaks down as one approaches the Faraday threshold. Then, the purely vertical bouncing states can be destabilized by the standing waves, giving rise to walking states, an effect to be treated in MBII. There, we couple the drop’s vertical dynamics, as described herein, to its horizontal dynamics. In order to determine the amplitude of the standing waves created and the tangential acceleration they impart to the drop, it is necessary to know the impact phase and the temporal dependence of the reaction force. The model for the vertical dynamics developed herein provides this information, the absence of which is responsible for the shortcomings of previous theoretical descriptions (Couder et al. 2005b; Eddi et al. 2011b) of this relatively subtle system.