问题 选择题

如图所示,有一箱装得很满的土豆,以一定的初速度在动摩擦因数为μ的水平地面上做匀减速运动,不计其他外力及空气阻力,则其中一个质量为m的土豆A受其他土豆对它的总作用力的大小是(    )

A.mg

B.μmg

C.mg

D.mg

答案

答案:C

由牛顿第二定律得箱和土豆的加速度为a=μg.土豆A受到重力mg和其他土豆的总作用力后,如图所示,它们的合力为ma,则F.

完形填空

t was December 25. Marie, a 13-year-old Australian girl, was happy. It was Christmas, and Marie’s mother was making a special cake. She put four small coins into the cake,  and then she baked (烤) it. The four coins were for good luck. After dinner Marie and her family ate the cake. They found three coins in the cake and put them on the table. Where was the fourth coin? It was missing, but Marie’s mother didn’t notice.

After Christmas, Marie got sick. She coughed, and she couldn’t speak. After six weeks she felt better, but she still couldn’t speak. Marie’s parents took her to hospital. Doctors at the hospital looked her over, and then said, "We are sorry, but we can’t help her." For 12 years Marie didn’t speak. She grew up, she got a job, and she got married. But she never spoke. One day,  when Marie was 25 years old,  she got a sore throat ( 嗓子疼 ) at work. She began to cough. She coughed up something small and black. What was it? Marie didn’t know. She took it to the hospital. A doctor at the hospital said, "This is a coin!" The doctor told Marie, "I think you can speak again." Marie went to a special doctor, and soon she could talk.

根据上面的短文内容回答问题。

小题1: What did Marie’s mother make at Christmas?

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小题2: What did Marie’s mother put some coins in the cake for? 

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小题3:How many coins did Marie’s family find after eating the Cake?

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小题4: How long didn’t Marie speak?

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小题5: Could Marie speak at last?

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单项选择题