在教科书“功的原理”一节中指出：“使用机械时，人们所做的功．都等于不用机械而直接用手所做的功．”而在“机械效率”一节中又指出：‘“有用功总小于总功，所以机械效率总小于1．”学过了这两节内容之后，初学者将认为这两个结论是互相矛盾的．现在我们以它们各自的题设条 In the section “The Principles of Work” in the textbook, it states: “When using machinery, people’s work is equal to the work done by hand without using machinery.” In the section “Mechanical Efficiency”, it is also stated that: “The useful work is always less than the total work, so the mechanical efficiency is always less than 1.” After learning these two sections, beginners will think that these two conclusions are contradictory. Now we set the bars up for their respective questions.