The benefits of rich tasks, project-based learning, and other inquiry-based approaches in terms of student understanding and engagement with mathematics are well documented. Such pedagogies are consistent with the development of mathematical proficiencies as described in the "Australian Curriculum: Mathematics" (Australian Curriculum Assessment and Reporting Authority [ACARA], 2013) and many teachers are keen to implement them. Basing mathematics teaching around projects and ill-structured problems, however, can be daunting for teachers who lack confidence in the mathematics content that they are teaching. This article describes an investigation of the relationship between the length of a pendulum and its period (time for one complete swing) conducted as part of a professional learning program with ten teachers in a Year 9-12 school attempting to teach the entire curriculum using project-based learning. The pendulum investigation was intended to illustrate the extent of the mathematics that could potentially be taught through a single investigation and to build teachers' confidence in exploring mathematics with their students.