问题 问答题

一机车汽笛频率为650Hz,机车以v=15m/s的速度观察者驶来,设空气中的声速是V=340m/s,观察者听到的声音频率为多少?

答案

机车静止时,汽笛声的波长为λ,由于机车(波源)向观察者运动,波长应减小vT,则单位时间内通过观察者的波数变为f′=

V
V-v
f=
340
340-15
×650=680Hz

即观察者听到的声音频率变大了,音调变高了.

阅读理解

阅读理解.

     One midnight a little girl woke up to pass water. She by herself got up and down her bed, walked to

the bedroom door and opened it. She looked outside and walked back, for it was so dark in the hall that

she feared.

     Her mother said, "Don't be afraid, honey. Take courage!"

     "What's courage?" She asked, running to her mother's bed.

     "Courage is the brave breath," her mother answered.

     "Mum, do you have courage?"

     "Certainly. "

     The girl held out her little hands, saying, "Mum, blow some of your breath of courage to me."

After her mother blew out two mouthfuls of breath into her little cold hands, the little girl clenched her

fists (握拳) nervously, afraid that the breath of courage would run away. Then with her fists clenched,

she walked out of the bedroom towards the bathroom with nothing to fear.

Her mother said to herself, "It will be nice if someone can blow some kind of breath to me. Then I can

hold it in my hands too when I feel terrified or lost. "

     In fact, mostly, what we are terrified of is nothing but the fear in our mind. Who we should defeat

(战胜) is nobody but ourselves.

1. The girl walked back because _____.

    A. the bathroom was too far away

    B. her mother wouldn't go with her

    C. she was too afraid of the dark

2. We learn from the reading that _____. 

    A. the girl went to the bathroom by herself

    B. the girl pretended to be terrified of nothing

    C. the girl's mother was able to blow brave breath

3. To stop "mother's breath" from running away the girl _____.

    A. held out her little hands

    B. clenched her fists

    C. talked to her mother

4. The writer mainly wants to tell us that _____.

    A. adults are always brave enough

    B. children learn to be brave quickly

    C. the fear in one's mind is the real problem

5. Which is the best title for the passage?

    A. The Fearless Mother

    B. One Lonely Girl

    C. The Breath of Courage

单项选择题