Conservative forces (gravity, spring, electrostatic) have path-independent work, zero work in closed loops, and allow PE to be defined. F = -dU/dx links force to PE.

Non-conservative forces (friction, air resistance) have path-dependent work, non-zero work in loops, and dissipate energy. No PE function exists for them.

When only conservative forces act: $E_{mech}$ = KE + PE = constant. When non-conservative forces also act: delta_E_{mech} = $W_{nc}$ $\frac{energy is lost to heat}{sound}$.

Identifying force type is crucial for choosing the right approach: energy conservation (conservative only) or the generalized work-energy theorem (any forces).