题干

长为10cm的弹簧在20N的力作用下伸长了0.5cm,那么在弹簧弹性限度内欲使弹簧伸长到13cm,应对弹簧施加多大的力?

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解:弹簧的伸长量和它受到的拉力成正比,当弹簧的长度是13厘米,则弹簧伸长13cm﹣10cm=3cm,受到20牛顿拉力时,则弹簧伸长0.5cm;也就是说伸长1cm,弹簧受到的拉力为40N,弹簧伸长3cm,应该用120N的力拉它。答:弹簧伸长到13cm,应当用120N的力拉它。

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    Hundreds of years ago, news was carried from place to place by people on foot or by horse. It took days, weeks and sometimes months for people to receive news. Now it is possible to send words and pictures around the world in seconds(秒). Billions of people learn about news stories of their own country and all over the world every day, either by watching TV or reading newspapers. Newspapers have been an important part of everyday life since the 18th century. Many countries have hundreds of different newspapers. How do newspaper editors(编辑) decide which news stories to print (印刷)? Why do they print some stories and not others? What makes a good newspaper story?

    Firstly, it is important to report new stories. TV stations can report news much faster than newspapers. Yet, newspapers give more about the same story. They may also look at the story in another way, or they may print completely (完全地) different stories to those on TV.

    Secondly, a news story has to be interesting and unusual. People don't want to read stories about everyday life As a result, many stories are about some kind of danger and seem to be "bad" news. For example, newspapers never print stories about planes landing safely, instead they print stories about plane accidents.

    Another factor(因素) is also very important in many news stories. Many people are interested in news in foreign countries, but more prefer to read stories about people, places and events (事件) in their own country. So the stories on the front page in Chinese newspapers are usually very different from the ones in British, French and American newspapers.