Abstract. While Strategic Intervention Materials (SIMs) are widely used, empirical research on their impact on learners’ understanding of projectile motion in Science 9 is limited. Existing studies typically focus on general science concepts, leaving a gap in evaluating the effectiveness of SIMs tailored to specific topics, such as motion in two dimensions. Many learners struggle with misconceptions about projectile motion, particularly viewing horizontal and vertical motion as interdependent rather than independent. Current SIMs often fail to address these misconceptions, contributing to underperformance. The study aimed to develop and validate instructional materials—specifically SIMs—to reteach topics deemed “least mastered” by learners in projectile motion. The developed SIMs were validated by experts through experimental and descriptive evaluative methods and were found to be highly valid and acceptable in both content and technical aspects. To evaluate their effectiveness, 136 heterogeneous Grade 9 learners from public (68) and private schools (68) were divided into control and experimental groups, with 34 learners in each group. The experimental group was taught using the developed SIMs, while the control group followed conventional teaching methods. Pre- and post-tests were administered, and data were analyzed using means and t-tests. The findings revealed a significant difference in academic achievement between the groups, with the experimental group demonstrating better results. This indicates that the SIMs effectively supported the learning process in projectile motion and helped learners master the least-mastered competencies. The study recommends continuous revision and expert validation of the SIMs to enhance their effectiveness. Feedback from learners’ post-usage is essential for ongoing improvement, and further research should explore additional least mastered competencies in Science 9. Additionally, professional development for Science teachers through seminars and training should be provided to support effective SIM implementation.

Keywords: Strategic Intervention Materials (SIMs); Projectile Motion; Science 9; Curriculum development; Instructional materials.