Newton's second law is one of the cornerstones of the introductory physics curriculum, but it can still trouble a large number of students well after its introduction, hobbling their ability to apply the concept to problem solving and to related concepts, such as momentum, circular motion, and orbits. While there are several possibilities for lab activities addressing the functional relationship among net force, mass, and acceleration, the qualitative understanding of the connection between forces and acceleration can still be lacking, leading to poor performance in problem solving and in assessments such as the Force Concept Inventory and Force and Motion Conceptual Evaluation. There is a need for strong conceptual understanding of the relationships between net force and acceleration and between acceleration and velocity in order to effectively address common force-motion misconceptions; there is a large literature concerning student understanding of force and motion.