Conservation of mechanical energy applies to a mechanical process in which
external force and non-conservative internal forces are absent.
There is no external force on the system. Hence, work by external force is
zero. There is no exchange or transfer of energy across the system.
Therefore, we use an isolated system to apply conservation of energy. We
should, however, note that transfer of energy from one form to another takes
place within the system, resulting from work done by internal force.
There is no “non-conservative” force like friction in the system. It means
that there is no change in thermal energy of the system. The internal forces
are only conservative force. This ensures that transfer of energy takes
place only between kinetic and potential energy of the isolated system.
Since potential energy is regained during the process, there is no
dissipation of energy.
As there is no dissipation of energy involved, the system represents the
most energy efficient reference for the particular process. One of the most
striking feature of this system is that the only forces working in the
system are conservative forces. This
has a great simplifying effect on the analysis. The work by conservative
force is independent of path and hence calculation of potential energy of
the system is path independent as well. The independence of path, in turn,
allows analysis of motion along paths, which are not straight.