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人教社版九年义务教育三年制初中物理第一册中,有这样一道习题:用动力臂是阻力臂2倍的杠杆,将重400牛的货物抬高20厘米,手向下压杠杆的力是220牛,手下降的高度是____厘米,人做的总功是____焦,有用功是____焦,这根杠杆的机械效率是____。____ 答案分别是40,88,80,91％。根据题目的条件,不难做出此答案。但对于这道题目的物理意义,却还有讨论的必要。 根据课本内容,“任何机械本身都受到重力作用,相对运动的零件间又存在摩擦,所以使用任何机械,除了做有用功外,都不可避免地要做额外功”。这样必然会有总功大于有用功,机械效率总小于1。实际利用机械时,施加的动力必然大于根据功的原理推算出来的动力。如果结合上述习题中的杠杆分析,结果又如何呢?
In the first edition of the junior high school physics for the nine-year compulsory education for the nine-year compulsory education school, there is an exercise that uses a power arm that is twice the lever and raises the weight of a 400 kilocalorie by 20 centimeters. The hand presses down on the lever. The force is 220 Niu. The height of the hand’s descent is ____ cm. The total work done by the man is ____Jiao. The useful work is ____Jiao. The mechanical efficiency of this lever is ____. The ____ answers are 40, 88, 80, 91% respectively. According to the conditions of the title, it is not difficult to make this answer. However, the physical significance of this topic is still discussed. According to the text of the textbook, “any machine itself is subject to gravity, and there are frictions between parts that move relatively. Therefore, using any machine, in addition to doing useful work, it will inevitably do extra work”. This will inevitably have more total work than useful work, and mechanical efficiency will always be less than one. In the actual use of machinery, the power applied must be greater than the power derived from the principle of work. If you combine the analysis of leverage in the above exercises, what is the result?