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Non-Conservative Forces and the Work-Energy Theorem

Idea: If there are non-conservative forces then mechanical energy is not conserved. We write,
W = Wnc + Wc = KEf - KEi      (7)
where Wc is the work done by conservative forces and Wnc is the work done by non-conservative forces. Since Wc = PEi - PEf we have,
Wnc = (KEf - KEi) + (PEf - PEi).      (8)
The work done by non-conservative forces is equal to the change in mechanical energy.