Which work is called useful, which is complete?

Useful work – work perfect only in relation to the working fluid, without taking into account the waste of work on the force of friction, and tools for performing useful work.
Full work is all work done by applied force.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.