Non-conservative forces are external forces that do work on an object, causing a change in its mechanical energy. These forces depend on the path taken by the object and not just its initial and final positions.
Related terms
Conservative forces: Forces that do not depend on the path taken by an object but only on its initial and final positions.
Work-energy theorem: A principle stating that the work done on an object is equal to its change in kinetic energy.
Frictional force: A non-conservative force that opposes motion between two surfaces in contact.