The present paper examines differences in students' conceptions of phenomena associated with basic concepts of simple projectile motion. The results presented focus on one aspect of projectile motion - students' understandings of kinematics concepts associated with the fundamental idea of speed as a function of distance and time (in vector terminology, velocity as a function of displacement and time). The special concern of this paper is to introduce the theoretical perspective on which the project is based and to discuss some common aspects of the problems.