This work presents the results of a study whose aim is to detect systematic errors about the concept of force among freshmen students. The researchers analysed the results of the Force Concept Inventory test, which was administered to two different groups of students. The results show that, although there were significant performance variations between the two groups, they, nonetheless, shared common incorrect answers that were consistently triggered by the same misconceptions. The analysis proposed in this paper could also be applied in other universities to reveal the students' "a priori" mindset in Newtonian mechanics and serve as a guideline for developing effective computer simulations or other tools. (Contains 2 tables and 5 figures.)