Quantum mechanics (QM) has become part of many secondary school curricula. These curricula often do not include the mathematical tools for a formal, mathematical introduction of QM. QM therefore needs to be taught at a more conceptual level, but making secondary school students understand counterintuitive QM concepts without introducing mathematical formalism is a challenge. In order to accept QM, students not only have to see the need of it, but also have to see that QM is understandable and logical. Dutch secondary school students are familiar with potential energy (PE) in the context of gravitational and elastic energy. Therefore, the introduction of QM by using the potential wells and tunneling with emphasis on students' prior knowledge of PE could be a way to make QM more understandable and logical. To explore this, we investigated the relation between the understanding of energy diagrams and the understanding of the potential well and tunneling. A module was created to promote students' understanding of PE in classical context. Then, a quasi-experimental intervention was used, in which the experimental group received additional lessons using the module on classical energy diagrams before being taught QM. Two tests were developed in order to determine students' understanding of PE and QM. The results of the tests showed that the experimental group not only had better understanding of PE diagrams, but also of QM even before they were being taught QM. Analysis of the tests also showed that there was a significant correlation between the understanding of PE diagrams and the understanding of QM. Therefore, the results of this study indicate that emphasis on PE can be used to reduce the gap between students' prior knowledge and QM.