The unquestionably central role of physics in the development of scientific literacy is undermined by its perceived difficulty. An investigation of high school students' use of the concepts of momentum and force suggests that, in the case of mechanics, the reason for physics' unpopularity and image as a "hard" subject is largely due to an incompatibility between the way it is taught in the standard model and students' cognitive representations. An analysis of high school students' understanding and use of force and momentum strongly implies that conservation laws should precede dynamics and kinematics in the physics curriculum due to the cognitive precedence of momentum over force. This conclusion is based on two findings: (a) students performed better at momentum than at force in pre-instructional activities; (b) an inversion in the order of introduction of topics shows that covering momentum before force is superior to the standard approach in enhancing students' understanding of mechanics. The study therefore provides a pedagogical rationale for physics instruction that is consistent with current learning theory.