Non-Conservative Forces and the Work-Energy Theorem

Idea: If there are non-conservative forces then mechanical energy is not conserved. We write,

W = W_nc + W_c = KE_f - KE_i (7)

where W_c is the work done by conservative forces and W_nc is the work done by non-conservative forces. Since W_c = PE_i - PE_f we have,

W_nc = (KE_f - KE_i) + (PE_f - PE_i). (8)

The work done by non-conservative forces is equal to the change in mechanical energy.