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问题 填空题

东风牌汽车在平直的公路上匀速行驶160m，所用时间为10s，若此时卡车所受的牵引力为5000N，则这段时间内牵引力所做的功为______J，牵引力的功率是______W．

答案

牵引力所做的功：

W=Fs=5000N×160m=8×105J；

牵引力做功功率：

P=

W
t
=
8×105J
10s
=8×104W．

故答案为：8×105，8×104．

选择题

下列装置中，没有用到电磁铁的是

A．电磁起重机

B．电磁继电器

C．电饭锅

D．电铃
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问答题

Passage 1 The US dollar value of Chinese exports increased at an average rate of almost 18 per cent per annum between 1978 and 1983, while imports increased by approximately 11 per cent per annum. As a result, the visible trade surplus rose sharply from US $1.4 billion in 1981 to US $4.4 billion in 1982 and US$3.7 billion in 1983. Exports grew much faster than imports during this period not only because of the p emphasis placed on exporting by China’s economic planners, but also because a number of industrial projects were postponed in 1979. Official recognition that foreign technology could play a major role in modernising the Chinese economy had caused imports to rise by more than 50 per cent in 1978 placing undue strain on the national economy. Grain imports have fallen sharply over the past few years—China became a net grain exporter in 1984—and in 1983 the country started to export soyabeans and cotton.

What is “a net grain exporter” Does it mean one who has never done any import
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