State the principle of conservation of mechanical energy in the presence of non-conservative forces.

(N/A) When non-conservative forces (such as friction or air resistance) act on a system,the total mechanical energy is not conserved. Instead,the work done by these non-conservative forces $(W_{nc})$ is equal to the change in the total mechanical energy of the system.
Mathematically,this is expressed as: $W_{nc} = \Delta E = \Delta K + \Delta U$,where $\Delta K$ is the change in kinetic energy and $\Delta U$ is the change in potential energy.
Alternatively,it can be written as: $E_f - E_i = W_{nc}$,where $E_f$ is the final mechanical energy and $E_i$ is the initial mechanical energy.
This indicates that the work done by non-conservative forces results in a change in the mechanical energy of the system,often dissipated as heat or sound.