Conservation of Energy
Energy can move between stores and cross system boundaries, but the total accounting remains conserved when every relevant form is included.
If friction reduces mechanical energy, where did the energy go?
Mechanical energy is only part of the ledger. Friction transfers energy into thermal stores. The broader total remains conserved.
If energy crosses the chosen system boundary, include that transfer explicitly.
K + U can decrease when energy becomes thermal.
Friction redirects part of the released potential energy here.
All three stores sum to the same total at every position.
If the surroundings carry energy away, the system's energy can change. Conservation applies to the larger closed accounting that includes the transfer.
Kinetic, potential, thermal, chemical, and other stores can increase or decrease while energy is redistributed.
K + U is not the same thing as total energy. Friction often makes the distinction visible.
When energy seems to disappear, widen the system or identify the transfer rather than abandoning conservation.