Not all forces behave the same way from an energy standpoint. Some have an associated potential energy — a reservoir into which energy can be deposited and from which it can be fully recovered — while others do not. This distinction is crucial: it determines whether, in the energy-exchange diagram, we must draw a potential-energy bubble or limit ourselves to a direct transfer between other bubbles.
Summary
Forces that admit a potential energy:
- Weight →
- Spring →
- Electrostatic force →
Forces without a potential energy:
- Friction
- String tension
- Normal reaction
The criterion separating the two families is conservativeness. The forces in the first group — weight, elastic force, Coulomb force — are conservative: the work they do in carrying a body from one point to another does not depend on the path taken, and this is precisely what allows a well-defined potential energy to be assigned to them. The forces in the second group do not enjoy this property. Friction dissipates energy irreversibly (it depends on how much distance is covered, not only on the endpoints), while the tension of a string and the normal reaction do not “store” anything: they transfer energy directly, without constituting a reservoir from which it can be recovered.
Links
Topics: Lavoro ed energia Concepts: Forze conservative · Energia potenziale gravitazionale · Energia termica da attrito
Related exercises: Worked exercise — block on an inclined plane with spring and friction · Worked exercise — minimum height for the loop · Problem — Block on an inclined plane with spring (equilibrium and oscillations)