A teacher of a secondary school explained to two groups of 14 and 15-year-old students the thermodynamic correspondent to the curriculum of physics and chemistry. This is an obligatory subject for all the students. They followed two different methods: one group followed the traditional methodology and other one the strategy of the cooperative learning. The material was facilitated to him to give the classes. This experience lasted two months. We used two motivational tests: a pretest and a posttest, to see the effect of the methodologies in the students. The group that followed cooperative learning increased very much his interest for the physics, and the other group did not change, even decreased it.