This work shows Physics Graduate students ways of thinking about the phenomenon of collisions and the physical quantities which are conserved in this process. Written questions and interviews about the described problems enabled us to analyse answers with expectations and explanations for the effect of simple and multiple collisions. Spontaneous models which were found, show fundamental aspects also obtained in other researches with high school students, although with different frequencies depending on the leve1 (graduate or secondary) and on the complexity of problems.