题干

崇左的夏天,烈日炎炎,酷热难耐.小明的妈妈为了遮挡热辣辣的阳光,在自行车的前把上安装了遮阳伞.可自从安装了这把伞后,骑车时总感觉自行车的前把好象被人向上提起,这是为什么呢?
上一题 下一题 0.0难度 选择题 更新时间:2018-01-09 03:58:39

答案(点此获取答案解析)

骑车时,等质量的空气在相同的时间内同时通过遮阳伞车的上表面和下表面,由于遮阳伞上表面弯曲,呈圆弧状,所以遮阳伞上部的空气流速快,压强小;下部的空气流动慢,压强大.在这两个压强差的作用下,遮阳伞受到一个向上的升力,感觉车把象被人向上提起

同类题3

阅读理解

    One of the most striking findings of a newly research in the UK is that of the people interviewed, one in two believes that it is becoming more difficult to meet someone to start a family with.

    Why are many finding it increasingly difficult to start close relationships? Does modem life really make it harder to fall in love? Or are we making it harder for ourselves? It is certainly the case today that contemporary couples benefit in different ways from relationships. Women no longer rely upon partners for money or status. A man doesn't expect his wife to be in (唯一的) charge of running his household and raising his children.

    But perhaps the knowledge that we can live perfectly well without a partnership means that it takes much more to persuade people to abandon their independence.

    In theory, finding a partner should be much simpler these days. Only a few generations ago, your choice of soul mate was limited by geography, social convention and family tradition. Although it was never clear,many marriages were essentially arranged. Now those barriers have been broken down. You can approach a builder or a brain surgeon in any bar in any city on any given evening. When the world is your oyster ( 牡蛎), you surely have a better chance of finding a pearl.

    But it seems that the old conventions have been replaced by the limitations of choice. The expectations of partners are raised to an unmanageable degree: good looks, impressive salary, kind to grandmother, and right socks. There is no room for error in the first impression.

    We think that a relationship can be perfect. If it isn't, it should be ended. We work to protect ourselves against future heartache and don't put in the hard emotional labor needed to build a strong relationship. Twelve-hour work at the office makes relaxed after-hours dating difficult. The cost of housing and child-raising creates pressure to have a stable income and Career before a life partnership.