Research on the learning and teaching of science is an important field for scholarly inquiry by faculty in science departments. Such research has proved to be an efficient means for improving the effectiveness of instruction in physics. A basic topic in introductory physics is used to illustrate how discipline-based education research has helped identify certain conceptual and reasoning difficulties that are common among university students and pre-university teachers. The results have been used to guide the design of instruction that has brought about a significant improvement in learning. The type of research illustrated requires a deep knowledge of physics and ready accessibility to students as they study that subject. Both of these conditions are usually present only in physics departments, not in departments in which the primary focus is on educational theory and methodology. Although the context of this paper is physics, analogies can readily be made to other sciences.