问题 选择题

一列横波沿x轴正方向传播,在ts与(t+0.8)s两时刻,在x轴上-3m~3m区间内的两波形图正好重合,如图所示.则下列说法中正确的是

A.质点振动周期一定为0.8s

B.该波的波速可能为10m/s

C.从t时刻开始计时,x=1m处的质点比x=-1m处的质点先到达波峰位置

D.在(t+0.4)s时刻,x=-2m处的质点位移可能为零

答案

答案:B

从图象上看出波长为4m,由题目条件可得周期为0.8n /s,故波速为5nm/s;

t时刻开始计时,x=1m处的质点向下运动,x=-1m处的质点向上运动,x=-1m的质点先到达波峰位置;0.4 s时间对应n/2个周期,x=-2m处的质点可能位于波谷或波峰不可能位于平衡位置。

阅读理解

阅读理解。

     Welcome to the Amazing kids Website! Here you can read about clever children all over the world.

     Gina, who is thirteen years old, got first prize in the "Young Cook of the Year" competition and is now

writing her own recipe (烹饪) book for kids. "I want it to be fun to read and easy enough for children to use,"

she says. Gina learned by observing (观察) her father, who's also good at cooking, in the kitchen. She wants

to encourage other children to cook because"everyone needs to eat, so it's a skill that will always be useful!"

     Fourteen-year-old Bren used to love running until he had a serious car accident three years ago. He lost part

of one leg and didn't even expect to walk again. Now he's an athlete who has recently completed a marathon.

"After my accident I wanted to give up," he says. "But now I'm happy to be alive and able to continue running."

    Helga has always been good at science, but she thought it was too difficult to become an inventor. Then

she heard about an accident that had happened when a car hit a train in her town. "That gave me an idea,"

says the clever sixteen-year-old girl. It took her over a year to invent a special machine that tells car drivers

when a train is coming. One company is already interested in selling it, so it may be in the shops soon!

1. How is Gina trying to encourage other children to cook?

A. By observing her father.

B. By writing a recipe book for kids.

C. By giving them delicious food to eat.

D. By getting first prize in the competition.

2. Why is it surprising that Bren is an athlete?

A. Because he can' t walk.

B. Because he lost part of one leg.

C. Because he gave up running.

D. Because he used to love running.

3. Which of the following is true about Helga?

A. She saw a car hit a train in her town.

B. She sold special machines to car drivers.

C. She invented a special machine for car drivers.

D. It is impossible for her to become an inventor.

4. Which would be the best title for the passage?

A. Amazing Kids, Special Abilities

B. How to Be a Good Student

C. Different People, Different Ideas

D. How to Get Living Skills

问答题 简答题