There is a huge body of research supporting a positive correlation between teachers' content knowledge and better pedagogical practice. In this context, this study revealed serious gaps in a cohort of preservice physics teachers' understanding of the simple pendulum. It also pointed to their poor grasp of some fundamental concepts in physics. Understanding the simple pendulum thoroughly requires the application of elementary kinetics of circular motion. This makes the scientifically accurate analysis of the simple pendulum motion reasonably complex, thereby offering potentially fruitful opportunities to probe the teachers' understanding. A questionnaire based on the simple pendulum and subsequent interviews were the research instruments. The findings demonstrate the gaps in the teachers' content knowledge, which could be addressed via initial teacher education and continuing professional development programmes for the teachers.