Let’s see the method at work in an elementary but instructive situation: a person standing on a platform resting on the floor. The objects involved are four — person, platform, floor and Earth — and our goal is to work out which and how many forces act on each of them, in particular on the person.
We draw each object as a circle and connect every interacting pair with a squiggle. The person has weight: there is therefore a weight squiggle between person and Earth. The platform also has weight: a second weight squiggle between platform and Earth. The person is in contact with the platform: a constraint squiggle (the normal reaction ). The platform rests on the floor: a constraint squiggle between platform and floor.
Interaction diagram: each object is a circle, each interaction a “squiggle”. Circling an object, the squiggles that cross the circle indicate the forces acting on it.
The decisive step is circling the person. We then count the squiggles that cross the boundary of the dashed circle: the weight squiggle (interaction with the Earth) exits, and the constraint squiggle (interaction with the platform) exits. There are exactly two forces on the person.
Key formula
Every squiggle crossing the circle corresponds to a force on the circled object: number of squiggles leaving = number of forces.
Note that the weight squiggle between the platform and the Earth, and the constraint squiggle between the platform and the floor, do not cross the person’s circle: they are interactions concerning the platform, not the person. The circle thus acts as an automatic filter, selecting only the forces relevant to the body of interest and excluding all the others. It is precisely this selection that makes the interaction diagram a foolproof tool for building the subsequent free-body diagram.
Links
Topics: Dinamica Concepts: Forza peso · Forza normale Skills: Diagramma di interazione
Related exercises: Problema — Bilancia in ascensore · Massa su piano liscio · Angolo di sopraelevazione della curva