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小明同学用如图1所示的滑轮组提土，他用80N的拉力把重力为120N的土从井中匀速提升5m，所花时间是20s．其中拉力做功W随时间t的变化图象如图2．求：

（1）有用功

（2）滑轮组的机械效率

（3）拉力的功率

（4）为了方便把土提上来，在图1中画出滑轮组的绕线．

答案

（1）W有=Gh=120N×5m=600J．

（2）把120N的土升高5m，用时20s，由图二知，拉力做功800J．

所以滑轮组的机械效率：η=

W有
W总
×100%=
600J
800J
×100%=75%．

（3）拉力功率：P=

W总
t
=
800J
20s
=40W．

（4）∵W=Fs，

∴绳子自由端移动的距离：s=

W总
F
=
800J
80N
=10m，

提升物体时绳子的段数：n=

s
h
=
10m
5m
=2，

所以滑轮组绕法如图．

答：（1）有用功是600J．

（2）滑轮组的机械效率是75%．

（3）拉力的功率是40W．

（4）如上图．

问答题

绘图题：请画出Y，d接线组别变压器相量图。
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单项选择题

Sustainable development is applied to just about everything from energy to clean water and economic growth, and as a result it has become difficult to question either the basic assumptions behind it or the way the concept is put to use. This is especially true in agriculture, where sustainable development is often taken as the sole measure of progress without a proper appreciation of historical and cultural perspectives.
To start with, it is important to remember that the nature of agriculture has changed markedly throughout history, and will continue to do so. Medieval agriculture in northern Europe fed, clothed and sheltered a predominantly rural society with a much lower population density than it is today. It had minimal effect on biodiversity, and any pollution it caused was typically localized. In terms of energy use and the nutrients captured in the product it was relatively inefficient.
Contrast this with farming since the start of the industrial revolution. Competition from overseas led farmers to specialize and increase yields. Throughout this period food became cheaper, safer and more reliable. However, these changes have also led to habitat loss.
What’s more, demand for animal products in developing countries is growing so fast that meeting it will require an extra 300 million tons of grain a year by 2050. Yet the growth of cities and industry is reducing the amount of water available for agriculture in many regions.
All this means that agriculture in the 21st century will have to be very different from how it was in the 20th. This will require radical thinking. For example, we need to move away from the idea that traditional practices are inevitably more sustainable than new ones. We also need to abandon the notion that agriculture can be "zero impact". The key will be to abandon the rather simple and static measures of sustainability, which centre on the need to maintain production without increasing damage. Instead we need a more dynamic interpretation, one that looks at the pros and cons of all the various ways land is used. There are many different agricultural performances besides food yield: energy use, environmental costs, water purity, carbon footprint and biodiversity. It is clear, for example, that the carbon of transporting tomatoes from Spain to the UK is less than that of producing them in the UK with additional heating and lighting. But we do not know whether lower carbon footprints will always be better for biodiversity. What is crucial is recognizing that sustainable agriculture is not just about sustainable food production.

How do people often measure progress in agriculture

A. By its productivity.
B. By its sustainability.
C. By its impact on the environment.
D. By its contribution to economic growth.
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