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什么是摩擦型带传动中的弹性滑动现象？可否避免？它对带传动有何影响？

答案

参考答案：

（1）由带的弹性变形而引起带与带轮之间的相对滑动现象称为弹性滑动。

（2）弹性滑动是带传动中不可避免的现象。

（3）它会引起以下后果：

①从动轮的圆周速度总是落后于主动轮的圆周速度，并随载荷变化而变化，导致此传动的传动比不准确。

②损失一部分能量，降低了传动效率，会使带的温度升高，并引起传动带磨损。

阅读理解

根据材料内容选择最佳答案，并将其标号填入题前括号内。

Dear Darcy,

     I have braces (牙套), so it's important that I brush my teeth. But I don't like to brush my teeth at all. Do you have any ideas for how to make me enjoy brushing? 

                                                                                                   —Annoying in Gainesville 

    Darcy: A few changes should help you feel better about brushing. Start by getting a new toothbrush in your

favorite color. A tasty toothpaste might make you smile.

Dear Darcy,

    My best friend has moved to the countryside and I'm finding school really hard without him. What shall I do?

                                                                                                   —Lonely in Los Angeles 

    Darcy: Face the situation bravely! What's more important, try to be friendly and find other kids that share

your interest. That way you and other kids will have much to talk about. Sure you'll make new friends with

each passing day.

Dear Darcy,

    My friend's birthday is coming soon. I want to give her something special but I haven't got enough money

and I'm not good at making things. What's your advice, please? 

                                                                                                   —Anxious

    Darcy: When your friend knows what you're thinking, I think she will be happy. Does your friend mind how

much money you will spend on the gift? Write a letter to her or buy her a gift with the money you can afford.

A true friend never cares how much a gift costs.

Dear Darcy,

    I listen to your program every night. I also have a problem. My cousin's dog has just had babies and I really

want to have one for a pet. My parents won't allow the baby dog in! I really need your help. 

                                                                                                   —Helpless in Seattle

    Darcy: Your parents sure have their reasons for not having a pet. Maybe they don't have time looking after it.

So have a talk with them and make them believe you can be a good pet owner.

1. According to Darcy, he who               is easier to make new friends. [ ]

A. likes writing letters to her

B. takes good care of himself

C. can share his interest with others

D. buys special gifts for others

2. Helpless in Seattle has a problem that             . [ ]

A. she has no money to buy a dog

B. she can't find her dog

C. she has no time looking after her dog

D. she can't have a dog at home

3. There is no information in the text about where               comes from. [ ]

A. Helpless

B. Anxious

C. Annoying

D. Lonely

4. Darcy most probably works at              . [ ]

A. a school

B. a hospital

C. a radio station

D. an airport

5. We can learn from the text that              . [ ]

A. they have the same problem

B. Annoying will change her braces

C. Anxious won't buy an expensive gift

D. Lonely also moves to the countryside

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The early expansion of the sugar industry was based on cane transported from two different parts of the Orient, first from India and second from the islands of the Southwest Pacific. From India, sugar cane was carried through the western regions of Asia into Arabia and later into the countries bordering on the Mediterranean. It was established in Sicily in about A. D. 703 and was carried to Spain in about A. D. 755. As early as A. D. 1,150, Spain had at least 75,000 acres of cane. During this early period sugar cane was grown largely to supply local wants. The Crusades were partly responsible for the further expansion and improvement of the sugar industry and for interesting Europeans in the use of sugar.
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