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 如图所示,通过安置在插座上的电流转换器,将电能转换为超声波,电动小车上的接收器收到超声波讯号,可将其能量转换为电能为电池充电.已知:接收器接收到的超声波功率与它和转换器间距离L的平方成反比,它将超声波能量转化为电能并储存在电池中的效率为50%.当小车与转换器间距离L0=1m时,接收器接收到的超声波功率P0=8W.当小车在平直路面上以0.2m/s的速度匀速行驶时,所消耗的电池电能的80%用于维持小车匀速行驶,小车受到的阻力为16N.

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    English is a very interesting language. It has borrowed words from many other languages. Immigrants(移民) coming into the United States have contributed many words to the language, which have kept their original pronunciation. “Coolie” and “kowtow” were taken from the Chinese language, “kamikaze from the Japanese, “shampoo” from India, “blitz” from German, “amigo” and “Los Angeles” from Spanish and so on.

    Many students have studied English for years, some as many as eight. However, some students still have difficulty in speaking fluent English. Some know many words but are unable to discern them when native speakers use them. In our Oral English classes we will focus on speaking and listening to native English speakers. For this reason, because we are trying to train your ears to hear English and your mouths to speak intelligible (易理解的) English, we will have a rule that ONLY ENGLISH will be spoken in our English classes. Anyone speaking Chinese in class will be required to pay a fine in order to encourage the speaking and understanding of English. If teachers enter a classroom and discover that anyone is speaking Chinese, they will require everyone in the room to pay the fine. It is everyone's job to follow the English-Only rule. It is for your benefit. It is because we want to accustom (使习惯于) your ears to hearing English.

    Other subjects may be learned only from books but the only way to learn a foreign language is to SPEAK IT! Students are often nervous about speaking in class at first but we hope to make the classes fun, so you will forget your nervousness and learn to speak out. Enjoy your classes.