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初二物理课本“机械效率”一节中有这样一道例题:用一个动滑轮把重400牛顿的货物提高2米,所用的力是250牛顿,求总功、有用功和这个动滑轮的机械效率。学生做过这道题以后产生这样一个疑问:使用动滑轮提高货物所做的功W_1=F×2h=250牛顿×4米=1000焦耳不用动滑轮,直接用手提高货物所做功W_2=Gh=400牛顿×2米=800焦耳(本题中 W_1和 W_2相当于 W_总和 W_有用)显然有 W_1>W_2,使用机械所做功不等于直接用手所做功,因此,能否说,考虑了额外阻力以后,功的原理就不成立了?也就是功的原理仅适用于不考虑额外阻力的理想机械。
In the second-year physics textbook “Mechanical Efficiency” section, there is such an example: the use of a moving pulley to increase the cargo weight of 400 Newtons by 2 meters, the force used is 250 Newton, the total work, useful work and the mechanical efficiency of the moving pulley . After the students made this question, they raised such a question: Use the moving pulley to increase the work done by the goods. W_1=F×2h=250 Newton×4 meters=1000 Joules Do not move the pulley and directly increase the work done by the hand. W2=Gh=400 Newton × 2m = 800 joules (W_1 and W_2 in this question are equivalent to W_sum W_ useful) Obviously there is W_1> W_2. The use of mechanical work is not equal to the work done by hand directly. Therefore, can we say that After the extra resistance, the principle of work is not established. That is, the principle of work applies only to ideal machines that do not consider additional resistance.