This study investigates the understanding of 18 first-year undergraduate students when simultaneously presented with two contrasting dynamical situations: the idealized (without air resistance) and real-world cases of balls being dropped or thrown. Previous work has shown that getting students to recognize flaws in their mental models helps them develop their understanding. Our students were better able to answer correctly the problems in idealized cases than the problems in real-world cases. For the real-world cases, the students understood the impact of air resistance on the object's size better than the impact of air resistance on objects of the same size but different mass. In follow-up interviews, the students reported that using the two different situations in the same test did indeed encourage them to think more carefully. By recognizing the need to include air resistance, they activated their appropriate mental "resources" to deal with the situations. We conclude that using contrasting situations (i.e., with and without an idealization) is a useful teaching tool.