0Work done by an external non-conservative force is equal to change in the mechanical energy of the system.
Final M. E - Initial M. E = W
If an external force does no work, then mechanical energy will be conserved.
(1) If no external force is acting on the system, then the system's momentum and mechanical energy will be conserved.
(2) If an external force is acting on the system, then :
(a) if work done by an external force is non 0 then both M. E and momentum will change.
(b) if work done by an external force is 0, then M. E will remain same I. e mechanical energy will be conserved, but momentum will change as an external force is acting on the system.
