Non-conservative systems are physical systems in which mechanical energy is not conserved. Energy can be converted from one form to another or lost due to external factors such as friction or air resistance.
Friction: The force that opposes motion between two surfaces in contact with each other.
Air resistance: The force exerted by air molecules on an object moving through the air.
Work-energy theorem: States that the work done on an object is equal to the change in its kinetic energy.