PIPES AND CISTERNS
The problems involving pipes and cisterns (taps and tanks) use the same concepts as discussed above. The pipes can be associated with men and filling up of the cistern can be associated with the work done. But, there is one difference between them. There can be some pipes which empty the cistern, in which case they are doing some negative work. Hence, a pipe filling the cistern is considered to be doing positive work and a pipe emptying the cistern is considered to be doing negative work.