问题 多选题

如图所示,沿x轴正方向传播的一列横波在某时刻的波形图为一正弦曲线,其波速为200m/s,下列说法正确的是(  )

A.从图示时刻开始,质点b比质点a先到平衡位置

B.从图示时刻开始,经0.01s质点a通过的路程为0.1m

C.若该波传播中遇到宽约3m的障碍物能发生明显的衍射现象

D.若该波波源从O点沿x轴正向运动,则在x=2000m处接收到的波的频率小于50Hz

答案

A、波沿x轴正方向传播,此时b质点振动方向向下,而a质点直接向下向平衡位置运动,则质点a比质点b先到平衡位置.故A错误.

    B、由图λ=4m,则T=

λ
v
=0.02s

      t=0.01s=

1
2
T,经0.01s质点a通过的路程为S=2A=0.1m.故B正确.

    C、λ=4m>3m,能发生明显的衍射现象.故C正确.

    D、若该波波源从O点沿x轴正向运动,则在x=2000m处接收到的波的频率升高,大于50Hz.故D错误.

故选C

阅读理解

阅读理解

     For years children in the industrial areas of Europe and America seldom left their smoky cities to see

the beauty of the countryside.Not that the woods and fields were always far away, but they were too far

from the city to permit people to make a round trip between morning and nightfall.What's more, factory

workers did not have enough money to send their children on country holidays away from home.

     In 1907 a young German schoolmaster had an idea which changed this state of affairs.He decided to

turn his little schoolhouse into a dormitory for the summer holidays.Anyone who brought his sleeping bag

and cooking equipment along could stay there for a very small amount of money.The idea was a success.A few years later the schoolhouse was far too small to hold so many young people who wanted to stay there.So, a dormitory was set up in an old castle nearby.This was the first Youth Hostel.

     Today young students and workers of every country can meet in the hostels and get to know each

other.Some spend a week or more in the same hostel, seeing the surrounding sights and meeting the people of the area.Other youths go on foot or by bicycle from place to place, spending a night or two in one

hostel, then going to the next.

     Sometimes an informal program will be organized after the meal, with dances, songs, or short talks

followed by a question period.One can learn a lot about other places, just by meeting people who come

from these places.For this reason, a few weeks spent "hostelling" can be just as useful a part of one's

education as classes in school.

1.  The reason why children in the industrial areas seldom left their smoky cities was that ________.

A.  they didn't like the beauty of the countryside at all

B.  their parents' income was not high enough to pay for the cost

C.  they couldn't make a round trip between morning and nightfall because of the distance

D.  Both B & C

2.  Paragraph 2 mainly tells us ________.

A.  how the first Youth Hostel began

B.  how to turn little schoolhouse into a dormitory for the summer holidays

C.  what the students should carry when spending summer holidays in the countryside

D.  the reason why spending summer holidays in the countryside

3.  According to the passage,, the following are the mentioned advantages of hostelling EXCEPT

________.

A.  enjoying the beauty of the countryside

B.  broadening their outlook and knowledge

C.  making new friends

D.  avoiding doing their homework

4.  用30词左右概括文章大意

    _______________________________________________________

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