“…Traditionally, introductory energy instruction in middle school focuses on forms of energy and transformations between those forms. Informed by the emphasis on energy transfers between systems in leading documents such as the Framework for K-12 Science Education (National Research Council, 2012) and similar proposals in the literature (Brewe, 2011;Ellse, 1988;Swackhamer, 2005), we have developed a middle school unit that focuses on the transfer of energy between systems and conceptualizes energy as unitary, i.e., no forms of energy are introduced (for a detailed description of the approach, see Nordine, Fortus, Lehavi, Neumann, & Krajcik, 2018). Following the recommendations in the Framework for K-12 Science Education (National Research Council, 2012), systems identify the part of the universe under investigation; thus systems can be single objects, fields, or collections of objects and/or fields.…”