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有这样一道中考选择题:“用不同机械效率的机械干同一件活,使用效率高的机械可以A.省力;B.省功;C.省距离;D.省时间。参考答案为[B]。命题人分析:功的原理”使用任何机械都不省功“成立的条件是无用阻力不计,机械效率η=1,才有 FS=Gh。本题所涉及机械的效率有高有低,即η=W_(有用)/W_(总).可得W_总=W_(有用)/η,而使用效率高的机械η大些,W_总必小些,即表示省了总功,故使用机械效率高的机械可以省功。但是,考生对”使用任何机械
There is such a multiple-choice test in the entrance examination: “The same work with different mechanical efficiency of the machinery, the use of high-efficiency machinery can be A. effort; B. provincial power; C. provincial distance; D. save time. The reference answer is [B Proposition person analysis: The principle of work ”The use of any machinery is not to save power " The establishment of the condition is that useless resistance does not count, the mechanical efficiency η = 1, there is FS = Gh. The efficiency of the machinery involved in this question are high and low That is, η=W_(useful)/W_(total). W_total=W_(useful)/η can be obtained, and the machine η with higher efficiency is larger, and W_ is always smaller, which means that the total work is saved. Therefore, the use of machinery with high mechanical efficiency can save power. However, candidates are