<p>The notion of momentum has received significant attention from sports psychologists. Recently, however, some researchers have introduced it to educational contexts and developed a psychological momentum perspective toward academic motivations. Different from other motivation theories, the psychological momentum theory mainly builds on Newtonian physics. It stresses the analogy between physical concepts (mass, inertia, and momentum) and psychological processes. While such a background brings several novel and appealing insights into academic motivations, as the theory is still new to the field, more validation work, such as those exploring its convergence and divergence with other established theories, is needed. Using self-determination theory as a complementary theory, the current study explored the convergence between the two theories by examining the association between self-determined motivations and two states of academic inertia (i.e., low-momentum state inertia [LMSI] and high momentum state inertia [HMSI]). The study also examined the two theories’ divergence by investigating how the two states of inertia predict academic engagement over and above self-determined motivations. </p> <p>Two hundred and six undergraduate students from a Midwestern university participated in this study. Regarding convergence, results provided mixed support for the hypothesis. No significant association was obtained between HMSI and all motivations; however, LMSI was negatively associated with intrinsic motivation and the relatively autonomy index but positively associated with amotivation (all to a weak-to-moderate extent). Regarding divergence, results demonstrated that inertia explained a moderate-yet-meaningful amount of variation in academic engagement, even after self-determined motivations are controlled for. Taken together, the results suggested the promise of PMT as a motivation theory. Based on the findings, implications and limitations of the study were further discussed.<b><br> </b></p>