Several researches in physics education have demonstrated the problematic status of teaching the subject of gravitation and weight and students' knowledge of these concepts. This paper presents findings of a study of students' knowledge following instruction within a changed conceptual framework of the weight concept in several 9th grade classes (N = 141). The experimental instruction addressed a representative set of physical situations designed to facilitate students' construction of adequate knowledge regarding weight and gravitational force. We checked the ability of middle school students to distinguish between these two concepts and the effectiveness of the operational definition of weight in helping students to account for novel physical situations. A written open-format qualitative questionnaire was used in the assessment that analyzed the data by means of quantitative (statistical) and qualitative (scheme-facets) tools. The same questionnaire was applied to 91 high school students exposed to the advanced placement level of teaching of the subject of gravitation and weight in Israeli schools. The results indicated an advantage of the experimental teaching with regard to students' pertinent conceptual knowledge.