Describing the motion in a vertical roller coaster loop requires a good understanding of Newton's laws, vectors and energy transformation. This paper describes how first-year students try to make sense of force and acceleration in this example of non-uniform circular motion, which was part of a written exam. In addition to an analysis of the exam solutions by about 60 students, a group interview was performed a couple of weeks later with four students, who had all passed the exam. The interview allowed the students to reflect on assumptions made and information missed.