机械效率是力学中的重点,也是一个难点,准确而迅速地求出机械效率的关键在于对有用功、额外功和总功的正确理解.判断有用功、额外功和总功,必须抓住工作目的.目的发生了改变,有用功和额外功也就发生了改变.有用功、额外功和总功的含义有用功可理解为一种“目的”功,即利用机械工作时,人们为达到某个目的所必须做的且对人们有用的功.例如,用滑轮组匀速提升重物时,使用机械的目的是提高重物,那么克服物体所受重力所做的功是有用功,即W_有=Gh(h为物体在竖直方向上升的高度).又如用滑轮组拉动重物在水平面上匀速移动时,使用机械的目的是水平移动重物,那么克服物体与 Mechanical efficiency is a key and difficult task in mechanics, and the key to finding out the mechanical efficiency accurately and quickly lies in the correct understanding of useful work, extra work and total work. Judging the useful work, extra work and total work, we must grasp the work Purpose. The purpose of the change has taken place, useful work and extra work also changed.Measurement of useful work, extra work and total work useful work can be understood as a “purpose ” work, that is, the use of mechanical work, people are For example, when a pulley is used to lift a heavy object at a constant speed, the purpose of using the machine is to increase the weight, then the work done to overcome the gravity of the object is a useful work, that is, W _ = Gh (h is the height of the object in the vertical direction.) Another example is the use of pulleys to pull the weight moving at a uniform level, the use of mechanical purpose is to move the weight horizontally, then overcome the object and