The present study investigated the underlying reasons for difficulties faced by students when they applied the concept of inertia across varying contexts. The participants of the study included five high school students. Data obtained from interviews were interpreted from the perspectives of the coordination class and epistemological framing theories. Analysis indicated that students demonstrated different reasoning patterns across the varying situations. Students' performances were influenced by their epistemological framings about problem solving and previous experiences related to learning physics. Students also revealed a lack of metacognitive awareness about their reasoning in related situations. The results also demonstrated that learning inertia is not a simple task as assumed. The study provides methodological and instructional implications for assessing and teaching inertia.