In the last decades, many education researchers have been trying to use computerized learning environments to enhance student learning. Without proper instructional supports and guidance, however, students often failed to acquire knowledge from computer-based learning activities. The objective of this study was to demonstrate how research-based formative assessment questions could be used as a framework for providing appropriate scaffolding in a computerized physics learning environment. To achieve this goal, an interactive physics tutor named APT (Assessment-based interactive Physics Tutor) was developed using Java and Open Source Physics. Examples of learning activities in APT were presented in order to highlight how formative assessment questions could be utilized to provide various instructional supports and guidance (e.g., prompting questions, descriptive statements, and computer simulations) tailored to the student's misconceptions. Educational implications of using APT were also discussed. (Contains 9 figures.)