  Physics Education Research has shown that the active methodologies are better
than traditional methodologies, the most popular are PODS and PI. However,
there are three problems that blur their effectiveness: first, the pre and
post-test do not allow to know what happens in the learning process; second,
the answers that average - performance students give to questions of a test are
influenced by the answers of the more advanced students; third, in many types
of research on PI the students do not experiment. In the present work, it is
exposed a research on the learning through PI with $42$ students over a small
physical concept (Bodies falling in air), the interaction between the students
was designed to correct the aforementioned problems. It was found a linear
evolution of the gain $G$, with high concentration in the correct model and it
was not observed peaks or interference.
