Summary — which bubble connects to what

The three families of forces — conservative, constraint and dissipative — are distinguished by how they connect bubbles and by the sign of the work they do. Collecting the rules into a single table lets you recognise, at a glance and given a force, which bubbles to draw and which way the arrow points.

Force	Bubbles connected	Sign of the work
Weight, spring, Coulomb, universal gravity (conservative)	Body’s $E_\text{cin}$ $\leftrightarrow$ interaction’s $E_\text{pot}$	$+$ if it enters $E_\text{cin}$, $-$ if it exits
Rope tension, constraint reaction (constraint)	$E_{\text{cin},1}$ $\leftrightarrow$ $E_{\text{cin},2}$; no $E_\text{pot}$	zero if the constraint is fixed; positive/negative on the two bodies if the constraint moves
Kinetic friction (dissipative)	Body’s $E_\text{cin}$ $\to$ $E_\text{term}$ (of the body or of the surface)	always negative (never positive!)

The table highlights the structural difference between the three families. Conservative forces are the only ones that introduce a potential-energy bubble, and their arrow is reversible (it can go either way). Constraint forces connect two kinetic energies and, in the common case of a fixed constraint, do no work at all. Kinetic friction is the only one with a one-way arrow: always from $E_\text{cin}$ towards $E_\text{term}$, never the other way. Keeping this map in mind is the fastest way to correctly build a bubble diagram starting from the interaction diagram.

Links

Topics: Lavoro ed energia Concepts: Forze conservative · Energia termica da attrito Skills: Bilancio energetico a bolle

Related exercises: Problema — Blocco spinto con attrito · Problema — Due ragazzi spingono una cassa · Esercizio svolto — blocco su piano inclinato con molla e attrito