We have already learned that Mechanical Energy (Kinetic Energy + Potential energy) remains constant or conserved When only conservative forces are present and there is no dissipative power
$K_F+ U_F = K_I+ U_I$
As discussed in case of spring mass system in reality dissipative forces like friction and air resistance are always present and some of the energy of the system gets dissipated in doing work against frictional forces. The work done by the friction is converted into heat energy by increasing the internal energy of the spring
This continues until system finally comes to rest
If one can somehow measure this energy ,the sum of KE,PE and this internal (or energy dissipated) would remain constant .This can be extended to all types of energy
$K.E + P.E + Dissipated Energy= Constant$
Energy can not be created or destroyed .It can only be transferred from one form to another form.Total energy in a closed system always remains constant
This is the law of conservation of energy although empirical one but it has never been found violated
Watch this tutorial for more information on How to solve work-energy problem
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