初中物理常涉及的简单机械有杠杆、滑轮、斜面和一些实际机械,如抽水机、起重机等,有关它们机械效率的计算则是初中物理的重点和难点,也是每年中考试卷的必考题.求解机械效率类问题的关键是正确区分各种情况下的有用功、总功和额外功.下面按照使用的机械类型分类讲解.一、杠杆的效率利用杠杆做功时,杠杆使物体升高,杠杆克服重力做的功为有用功,即W(有用)=Gh;动力对杠杆做的功是总功,即W_(总)=Fs;克服摩擦以及杠杆自重做的功是 Physics junior high school physics often involve levers, pulleys, bevels and some of the actual machinery, such as pumps, cranes, etc., on the calculation of their mechanical efficiency is the focus and difficulty of junior high school physics, but also the exam papers in the annual exam. The key to class problems is to correctly distinguish the useful work, the total work and the extra work in each case. The following classification according to the use of mechanical type to explain.First, the efficiency of leverageLeverage to do work, leverage to make objects, leverage to overcome gravity Of the work is useful work, that W (useful) = Gh; power work on the lever is the total work, that W_ (total) = Fs; work to overcome the friction and leverage is done