问题 选择题

如图12-6-16所示,S1、S2是振动情况完全相同的两个机械波源,振幅为A,a、b、c三点分别位于S1、S2连线的中垂线上,且ab=bc,某时刻a是两列波的波峰相遇点,c是两列波的波谷相遇点,则(   )

图12-6-16

A.a处质点的位移始终为2A

B.c处质点的位移始终为-2A

C.b处质点的振幅为2A

D.c处质点的振幅为2A

答案

CD

S1、S2是振动情况完全相同的振源,则到两波源距离之差等于半波长偶数倍的点振动加强,振幅是两列波振幅之和,即为2A;到两波源距离之差等于半波长奇数倍的振动减弱,振幅为零,两波源连线中垂线上的所有点到两波源距离之差为零,符合振动加强条件,故C、D两项正确.振动加强只是振动的振幅最大,位移在-2A—2A之间不断变化,故A、B两项错误.

阅读理解

阅读理解

     Throughout the Depression (经济萧条) years,my brothers,sisters and I often heard our granny say,

"One of these days,my ship is coming in."Of course Granny's ship was just a dream-her escape from the

reality of being poor most of her life.But to us it was very real.

     Our family got by with the barest (最少) of necessities.There rarely was enough left over for special

desires.So if we wished for other things,we told Granny about them,and she would plan for them to be

aboard her ship.

      At times we kids worried greatly as to how the ship was going to arrive at Granny's house in the

deep pine woods of East Texas,miles from any waterway.One day when Granny was sitting with us on

her front porch,we asked her how she expected the ship to come.Shifting in her rocking chair,she

expected the ship to come.Shifting in her rocking chair,she pointed to the beautiful clouds floating in the

bright blue sky.

      "Why,children," she explained,"Granny's ship has big beautiful sails.Some day it will rise up on clouds

like those,and come floating right up to the front door."

     As we grew older,our childish wishes for BB guns,baby dolls and mohair sweaters were replaced by

dreams of an education,a job or a lovely wedding.We told Granny of those goals and she put each one

aboard her ship.

     Granny has been gone a long time now,but I still remember how we sat with her on the porch scanning

the skies.She never told us the name of her ship,but I think perhaps it was "Hope",for Granny told us to

have patience,to reach beyond our limitation,and many of our dreams and wishes did come true.

      Even now,sometimes,when the skies are brilliantly blue,with soft puffy clouds sailing along with the

breeze (微风),I look up-to see Granny's ship.Its sails are full and proud.It is a beautiful ship,carrying a

wonderful cargo of many packages of many sizes-all marked "From Granny.With Love".

1. From the text,we know________.

A. Granny has been dead for a long time

B. Granny used to have unpractical ideas about life

C. Granny's ship arrived at last

D. Granny has moved to the city

2. We can infer from the text that the writer's family lived________.

A. in a city  

B. in Granny's ship

C. in a mountainous village 

 D. at the seaside

3. What does the underlined word "them" in the second paragraph refer to?

A. Our necessities.  

B. Our wishes.

C. The children.  

D. Package.

单项选择题