Physical interaction happens during a shared moment of a minimum of two actors (humans, animals, computers, etc.). They are related by conscious or unconscious intentions of gesture/action that create a context in which the sum of the actions and intentions builds an active relationship between them. The nature of the physical interaction can vary depending on the level of complexity of the intentions and of the possibilities of results that can emerge from the active relationship in between the two actors.

Crawford’s example of the refrigerator light is a good one. Opening the refrigerator door is interactive but at a very low level because the possibilities of results generated by the interaction moment are very limited.

I like to think physical interaction an ergonomics of space and time in motion. Ergonomics is an applied science that helps build equipment and devices that fit the human body and enhance its capacities. Physical interaction is the complementary side of ergonomics: it describes the gestures and interactions with something, the actor. The design physical interaction we need to consider various things:

• Analyse the nature of the interacting actors: are they humans, animals, computers?;
• Identify the intention of the gestures involved in the physical interaction: what is the interactive “story” that is being told?;
• Identify the different steps and actions that can be accomplish by each of the various actors involved;
• Compare and identify all the possibilities of interaction and results that can be generated by the physical interaction moment.