FORCES are acting in the system. \[\Rightarrow \] \[{{W}_{
EXT
}}=\Delta
U
\] (If
BODY
moves without acceleration) \[{{W}_{ext}}=\Delta U+\Delta KE\] (if body moves with acceleration) \[\Rightarrow \] \[{{W}_{\text{conservative}}}=-\Delta U\] (
ALWAYS
)