问题 多选题

用细绳拴一个质量为m的小球,小球将一固定在墙上的水平轻质弹簧压缩了x(小球与弹簧不拴连),如图所示.将细绳剪断后(  )

A.小球立即获得

kx
m
加速度

B.小球在细绳剪断瞬间起开始做平抛运动

C.小球落地的时间等于

2h
g

D.小球落地的速度大于

2gh

答案

A、细绳剪断前受重力、弹力和拉力,三力平衡,故弹力和重力的合力为:F=

(kx)2+G2
;剪断细线后,弹力和重力不变,故合力为F=
(kx)2+G2
=ma,故a≠
kx
m
,故A错误;

B、将细绳烧断后,小球受到球的重力和弹簧的弹力的共同的作用,合力斜向右下方,并不是只有重力的作用,所以不是平抛运动,故B错误.

C、小球竖直方向只受重力,做自由落体运动,故小球落地的时间等于

2h
g
,故C正确;

D、如果不受弹簧弹力,小球落地速度等于

2gh
,有弹簧弹力做功,故落地速度大于
2gh
,故D错误;

故选CD.

阅读理解

In the summer of 1978 an English man named Steven was driving his tractor through a field of wheat when he discovered something strange. Some of his wheat was lying on the ground. The flattened wheat formed(形成)a circle about six meters across . Around this circle were four smaller circles of flattened wheat.

  Three years later a farmer who lived nearby discovered almost the same circles in one of his fields. These circles were larger—nearly 15 meters across . That same year , another English farmer discovered three circles of flattened wheat on his land—one large circle between two small circles. During the following years, farmers in England found circles in their fields more and more often.

  The circles are called “crop circles” because they usually appear in fields of wheat or corn. The wheat in the circles lies on the ground but is never broken; it keeps on growing, and the farmers can later harvest it. Farmers always discover the crop circles in the morning, so the circles probably form at night. They appear only in the months from May to September. What causes the crop circles?

  At first, people thought that some kids were making them as a joke, or that farmers were making them to attract tourists.(In fact, in 1991, two men said they made the circles themselves, but many scientists don’t believe them.) People tried to copy them: They tried to make circles exactly like the ones the farmers had found. They couldn’t do it. They couldn’t enter a field of wheat without leaving tracks, and they couldn’t flatten the wheat without breaking it.

  Several times people reported seeing stranger objects near the fields where crop circles later appeared. Many people believe that these crop circles are the messages sent by living things from outer space(外层空间)or the marks left by their spaceships.

  Scientists who have studied the crop circles try to find out what causes them. In the summer of 1990 some scientists spent three weeks in the part of England where many circles have appeared. They had all the latest high-tech equipment(最新高科技设备). The equipment—worth 1.8 million dollars—got nothing. But one night, as the scientists were watching a field, crop circles formed in the field behind them, which were quite different from the others. The scientists had neither seen nor heard anything.

  When Steven discovered the crop circles on his land in 1978, he said, “It was just like something that had landed in the field from the air and gone back up again. I don’t know what to make of these things.”

  Crop circles have appeared in England, Japan, the United States and Russia. Experts from all over the world have studied them, and they say what Steven said: They don’t know what to make of these things.

小题1:Which picture is probably the one formed in the field behind the scientists?

小题2:“Flattened wheat” means ___________.

    

A.broken wheat

B.lying wheat

  

C.harvested wheat

D.growing wheat小题3:The passage is mainly about something ___________ .

A.that is done by living things from outer space

B.that cannot be solved but found all around the world

C.that cannot be made clear or understood

D.that is discovered and copied by the farmers

选择题