Because it
is a starting point for the topic WAVES, one of the most interesting
topics in Physics. (We will develop soon some of the interesting topics
related to Waves: Music, Resonance, …)
AND…
Students went to the lab to perform their traditional experimental
work. They had a guide to study the relationship between the amplitude,
the mass and the length with the period.
But then they found a “new” system: the Physical pendulum. A strange body had a movement.
And students had to think about it. They were asked to think about the new system, to think about their motion.
Could they affirm that this system had a SHM? Why?
And their answers:
Finally: I had the surprise.
Most of the students found it difficult to apply the concept of
simple harmonic motion to a system not deeply studied in class, such as
the physical pendulum.
Students
did not consider that the model of simple harmonic motion was related
to the direct proportion between the force (or torque) and the
displacement, even though it was discussed.
They thought that the important point for the model was the negligible mass.
Is it possible to build the model only if the mass is punctual?
Besides they thought that
the physical pendulum didn’t have a SHM because the energy wasn’t
conserved. This point wasn’t a limitation to the model. In that case the motion is harmonic but also damped.
That is, the limitation of the model is not related to the energy conservation but to the hypothesis of direct proportion mentioned before.
We don’t know, at this step
of our analysis, if the students’ confusion is due to a problem of
acquiring the two cognitive tools mentioned or to a didactic problem.
We consider it is necessary to clarify the differences among hypotheses in class. This clarification would
probably facilitate the discussion on the acquisition of cognitive
tools studied in this work.
However, we believe that, in spite of the difficulties
encountered in the acquisition of cognitive tools during Physics
learning processes, it was possible to start thinking about the importance of taking into account the differences in the formulated hypotheses when we teach these topics.
I would like to
thank the assistant teachers of practical lessons of the Physics
Department of Colegio Nacional de Buenos Aires (CNBA) [Buenos Aires
National High School] and especially to Andrea Povis, Carlos Elizalde y
Mariela Josebachuili for their help to discuss these ideas, and the
analysis of the educational experience, as well as their enthusiastic
work with their students. In addition, I would like to thank Gabriela
Schenoni, UTN [National Technological University] professor for their
useful comments about the article in Spanish.
Besides, thank Ignacio , my brother, for their comments about the complete article in English.
It is possible to find more information about cognitive tools and kind of understanding in “The educated mind” (Kieran Egan, 1997) and in the following web sites depending on the IERG.(International education research group)
www.ierg.net
http://www.ierg.net/about/bigideas.html
http://www.ierg.net/about/cog_thoughts.html
http://www.ierg.net/about/cog_theory.html