This study analyzes the operational knowledge of the weight concept of high school students after two educational levels: Introductory and advanced physics courses. The results show that most of the students at both levels construct alternative understanding of weight, which can be represented in operational schemes. The study shows that apparent and true weight concepts are poorly assimilated by most of the advanced placement students. Students' confusion is interpreted as reflecting the misfit of the weight concept image by students and the weight concept definition currently adopted by most of U.S. physics textbooks and identifying weight with the gravitational force. The origins of the shortcomings of the alternative knowledge about weight are listed and discussed. An alternative, operational definition of weight, separating it from the gravitational action at a distance, might be preferable in the educational context. Students' intuitive weight considerations have close historical parallels, which could elucidate the epistemological roots of their alternative knowledge.
