机械效率是初中物理的重要内容 ,也是中考的热点之一 ,求解的关键 :一是理解使用机械的目的、方法 ,二是确定有用功、额外功 .下面举例分析 .图 1例 1 　用图 1所示的滑轮组提起重 63 0N的物体 ,绳子自由端的拉力F为 3 0 0N ,重物被匀速提升 2m ,求 :(1 )滑轮组的机械效率 ;(2 Mechanical efficiency is an important part of junior high school physics, and it is also one of the hot spots in the examination. The key to solving is to understand the purpose and method of using the machine, and to determine the useful work and extra work. The following example is used for analysis. Figure 1 Example 1 Figure 1 The pulley shown lifts an object weighing 630N. The free end of the rope has a pull force F of 300N and the weight is raised at a uniform speed of 2m. Find the mechanical efficiency of the (1) pulley block; (2)