The purpose of this study is to propose a process model to explain elementary school students' solutions of three simple problems in elementary mechanics. Forty eight 5th grade students were given three drawings depicting objects of various sizes in different kinetic states or being pushed by a human agent. They were asked to say whether a force was being exerted on the objects and to explain why. A process model has been proposed to explain students' answers to the three questions. The innovation of the process model is that it attempts to link two levels of representation: A semantic level, where a concept is analysed in terms of the presuppositions, beliefs, and mental models that underlie it and a syntactic level that specifies how concepts are related to other concepts in hierarchical categories. The work has been validated by a computer model designed by the AI team (Vosniadou, Kayser, Champesme, loannides & Dimitrakopoulou, in press).