When only conservative forces act on and within a system, what quantity remains constant throughout the process?

1. Starting from the work-energy theorem and substituting Wc = negative delta PE for conservative forces gives delta KE plus delta PE equal to zero. 2. That result means KE plus PE, the mechanical energy, stays constant throughout the process whenever only conservative forces act. 3. Kinetic energy and potential energy individually do change, converting back and forth into each other, so neither one alone stays fixed. 4. Individual forces can still do nonzero work on their own; it is only the total mechanical energy, not each separate force's work, that remains constant here. _Source: OpenStax College Physics (CC BY 4.0), Ch 7 "Work, Energy, and Energy Resources", section 7.4 Conservative Forces and Potential Energy_