Within the discussion of the development of commonsense theories of motion, recent research has established that throughout middle childhood, reasoning about motion down inclines changes with increasing age. To investigate this shift in more detail, this study investigated 5- to 11-year-old children's understanding of motion down curved slopes, addressing the changing interaction of horizontal and vertical dimensions along a single trajectory. This allows for a close examination of the notion of children's ability to integrate horizontal and vertical motion knowledge as opposed to encountering a third conceptual-reasoning component within the commonsense theories framework. Children (N = 115) participated in one of three motion conditions--straight incline, convex incline, and concave incline. They predicted motions of two balls (heavy versus light) down the slopes, addressing comparisons between sections of the trajectory (shallow, intermediate, and steep incline). The results suggest that children do appear to integrate information about horizontal and vertical motion when judging motion down inclines, arguing for a two-component commonsense theory system. The results are situated within the context of conceptual knowledge structures and potential implications for educational practice are discussed.