  We present a critical analysis of the classical approaches to energy
subjects, based on the work-energy theorem and the conservation of mechanical
energy proposed in the courses of the first years of tertiary education. We
show how these approaches present a series of inconsistencies and errors that
are a source of conceptual difficulties among students. We then analyze a
modern treatment of mechanical courses based on the results of research in
physics education over the last 40 years. We place special emphasis on the
principle of conservation of energy as one of the fundamental principles of
nature, prioritizing the concepts of system, surrounding, and energy transfer
and transformation.
