Common Themes


Scientists explain their observations of the world in terms of interactions between objects. In the PET course students also examine many different phenomena that are attributed to interactions between pairs of objects. These interactions are classified according to certain identifying characteristics, which are shown in this chart.


Throughout the PET course students analyze interactions in terms of the idea of the transfer of energy between the interacting objects. During an interaction one object serves an energy source (in which a certain type of energy decreases) and the other as an energy receiver (in which a corresponding increase in a certain type of energy occurs). This energy framework is represented in an energy diagram, like that shown below for a contact interaction between a person’s hand and a cart:

It should be noted that the terminology used for the various types of energy encountered in the course is not necessarily that used by the scientific community. However, these terms were carefully chosen to be meaningful to students for pedagogical reasons.


The construction and evaluation of scientific explanations of phenomena are skills that many students initially find quite difficult. The approach taken in the PET course is to scaffold these skills strongly at first, and to gradually fade this support as the course progresses. Students are prompted to first perform an analysis in which they identify the parameters of the interaction and construct an energy or force diagram, and then write the explanation itself.

Early explanation activities also ask students to evaluate previously written explanations using the explicit guidelines provided to them.  They use the same structure to explicitly evaluate their own explanations and those of their classmates. Later in the course, this supporting structure is removed as all the steps in the procedure become more familiar.