Work is the amount of energy a task requires - duration is the length of
time it takes to do it. It takes an hour for 1 man to paint 10 square feet
of wall and we have 100 square feet to paint. Thus if Bill works at full
capacity it will take him 10 hours to paint it, ie, the painting task
requires 10 man-hours. But if he's also got to do something else at the
same time, he can't devote his full attention to the painting. Maybe he can
only devote half his attention - 50% effort - to the painting task. It
still will require 10 man-hours of work but because he's distracted he only
gets 1/2 man-hour worth accomplished for each hour he puts in on it, thus
though the work is still 10 man-hours, the duration becomes 20 hours. On
the other hand, if we can get another guy to help him and they both work
full speed - 100% effort each - we can get 2 man-hours worth of work done
for every hour of clock time they spend on it and the duration becomes 5
hours to get the same 10 man-hours of work accomplished.
