在学习“简单机械”时,关于机械效率的知识是难点,而解决这个难点的关键在于准确、熟练区分有用功、额外功和总功.辨别三种功对有用功、额外功和总功的正确理解是我们学习机械效率的前提条件.1.有用功.有用功是指人们需要的、有价值的功.如我们要提水上楼,目的是提水,因此我们克服水所受重力做的功为有用功.2.额外功.额外功是指并非我们需要,但 When learning “Simple Machinery”, knowledge about mechanical efficiency is a difficult point, and the key to solving this difficulty lies in accurately and skillfully distinguishing between useful work, extra work, and total work. The correct understanding of work is a prerequisite for us to learn the mechanical efficiency .1 useful work useful work refers to people need, valuable work .If we want to mention the water upstairs, the purpose is to raise water, so we overcome the gravity of water The work done is useful work .2 extra work .Extra work is not what we need, but