人们在使用机械做功时,除做有用功外,还要克服机械中的摩擦做功,或克服机械本身重力做额外功,这部分功称为额外功,有用功与额外功之和称为总功.由于有额外功,机械做功要存在机械效率,机械效率的公式为η=W有用/W总.在中考题中,关于机械做功的考题比较典型,有些难度,只要找出有用功和总功这一关键,难题也就容易解决.如何找出和求得有用功与总功呢?求总功W总时,要看机械运动的原因,方法大致 People use mechanical work, in addition to doing useful work, but also to overcome friction in the machinery to do work, or to overcome the mechanical gravity itself to do extra work, this part of the work as additional work, useful work and extra work called the total work of Gong . Because there is extra work, mechanical work to have mechanical efficiency, mechanical efficiency formula η = W useful / W total. In the test questions, the mechanical work on the test questions are more typical, some difficulty, as long as the useful work to find and total work This key, the problem is also easy to solve.How to find and seek useful work and total power? Seeking total work W, depends on the reasons for the mechanical movement, the general approach