Suppose a system of particles is subject to internal forces, some of which are conservative and some of which are non-conservative. Let $\Delta E$ be the change in mechanical energy of the system as it undergoes a change in configuration and $W_{nc} $ be the work done by the non-conservative forces in that change in configuration. It can be shown that
$\Delta E=W_{nc} $
Rewriting the equation above we get
$\Delta E-W_{nc}=0 $
Is there any special name given to the quantity $\Delta E-W_{nc} $? Further, what's the physical significance of the second equation? Surely, it's a conservation law, but of what?
