The mechanical energy of a system of objects is conserved only when the work done by the resultant external force is zero.
Mechanical energy is a special class of energy that an object contains due to its relative position or motion/movement level.
This type of energy (mechanical energy) is associated with both kinetic motion energy and potential (stored) energy.
In conclusion, the mechanical energy of a system of objects is conserved only when the work done by the resultant external force is zero.
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