问题 计算题

如图所示,声源S和观察者A都沿x轴正方向运动,相对于地面的速率分别为vsvA.空气中声音传播的速率为vp,设vs<vp,vA<vp,空气相对于地面没有流动.

(1)若声源相继发出两个声信号.时间间隔为Δt,请根据发出的这两个声信号从声源传播到观察者的过程.确定观察者接收到这两个声信号的时间间隔Δt'.

(2)请利用(1)的结果,推导此情形下观察者接收到的声波频率与声源发出的声波频率间

 
的关系式.

 

答案

(1)        (2)

(1)设t1、t2为声源S发出两个信号的时刻,为观察者接收到两个信号的时刻。

则第一个信号经过(-t1)时间被观察者A接收到,第二个信号经过(-t2)时间

被观察者A接收到。且t2-t1=△t  =△t′

 
设声源发出第一个信号时,S、A两点间的距离为L,两个声信号从声源传播到观察者的过程中,它们运动的距离关系如图所示,可得

  

由以上各式,得

(2)设声源发出声波的振动周期为T,这样,由以上结论,观察者接收到的声波振动

的周期T为 

由此可得,观察者接收到的声波频率与声源发出声波频率间的关系为

阅读理解

阅读理解

     Everyday, 340 million people speak it. One billion people are learning it and it is said that by 2050,

half of the world's population will be using it. What are we talking about? That global language-English.

     The English language started in Britain in the 5th century. It is a mixed language. It was built up when

German. Scandinavian and French invaders settled in England and created a common language for

communication.

     Today it is the official language of the UK, the USA, Australia, New Zealand, Canada, South Africa

and Ireland as well as many islands in the Caribbean . Many other countries and regions use it for politics

and business, for example, India. Pakistan, Nigeria and the Philippines. English is also one of the official

languages of Hong Kong.

     But global advertising and pop music mean that in most countries, you will see or hear some English.

Thanks to McDonalds, we all know about "burgers". "fries" and "milkshakes". Songs by Madonna,

Britney Spears and Celine Dion are in English. We can sing along, even if we do not understand what we

are singing!

     English is a messy (杂乱的) language. Every year, dictionaries include new words that talk about

popular culture, for example, computer-related words such as "blogging", "download" and "chatroom".

Also included are words that teenagers use. Who does not know "cool", "OK" and "hello" ?

     Other languages also influence English. Many English words come from French. Words like "cafe"

and expressions like "c'est la vie" (that is life) are all part of the English language. On the other hand, the

French language includes English words like "le weekend" and "le camping". German words are also part

of English. Words like "kindergarten" come from the German language.

     Recently, British people have become interested in "yoga". But the word comes from an ancient Hindu

language in India.

1. The English language has a history of ______.

A. over 2000 years

B. over 500 years  

C. over 1500 years

D. over 1000 years

2. The underlined expression "thanks to " can be replaced by ______.

A. because of

B. thankful to    

C. not until

D. as if.

3. Which of the following statements is true about the language of English ?

A. It has been changing all the time.  

B. It has borrowed words from all the other languages.

C. French words are used by the English because dictionaries have French words.

D. Singers and film stars have the greatest influence on language.

4. How many people in the world are using English now?

A. One billion people                      

B. 340 million

C. almost all the people in the world.

D. not mentioned above, but the number is growing rapidly.

5. Many countries and regions use it for politics and business except______.

A. the USA    

B. Nigeria      

C. the Philippines    

D. Norway

问答题 简答题