The first calculus course in the province of Quebec (Canada) is taught in the first year of college (17-18 year-old students) before university. Statistics show that this course is the most difficult one for students at the collegial level and that it prompts many to drop out of school. The literature has highlighted the cognitive problems students have with integrating concepts related to pre-calculus and their problems in learning calculus concepts related to infinity. In the current century, a new variable is added to the teaching of introductory calculus courses, namely, the introduction of situational problems as a way to generate new knowledge. This new approach to teaching is mainly related to modelling processes in the generation of knowledge. In this paper, we show the cognitive problems students have in learning calculus when solving situational problems. More precisely, we observe students' mathematical activity as they solve an open-ended task related to speed. We analyze their use of different representations in the process of modelling the situation, the evolution of these representations when students work in a collaborative learning approach, and their construction of the concept of the derivative.