关于机械效率的知识 ,是学生难掌握的一个知识点 ,特别是对同一机械的效率是个“变数”印象不深 ,在实际解题中屡屡出错。1　例 1　用滑轮组匀速提起重为 1 0 0 0N的物体 ,绳子自由端的拉力为 31 2 .5N ,此时滑轮组的机械效率为 80 %,不计摩擦和绳重 ,求①需几根绳了承担重物 ; The knowledge of mechanical efficiency is a knowledge point difficult for students to grasp, especially for the efficiency of the same machine is a “variable” is not deep impression, in the practical problem-solving often mistakes. 1 Example 1 Pulling an object with a weight of 1000N using a pulley pulley at a uniform speed, the free end of the rope has a pulling force of 31 2 .5N. At this time, the mechanical efficiency of the pulley group is 80%. The friction and the rope weight are not taken into consideration. Heavy objects ;