问题 选择题

如图所示,O1为皮带传动的主动轮的轴心,轮半径为r1,O2为从动轮的轴心,轮半径为r3; r2为固定在从动轮上的小轮半径.已知r3=2r1,r2=1.5r1.A、B和C分别是3个轮边缘上的点,质点A、B、C的向心加速度之比是(  )

A.1:2:3

B.2:4:3

C.8:4:3

D.3:6:2

答案

对于A与B,由于皮带不打滑,线速度大小相等,即vA=vB.由v=ωr得ωA:ωB=r3:r1=2:1.

    对于B与C,绕同一转轴转动,角速度相等,即ωBC

则ωA:ωB:ωC=2:1:1

根据a=ω2r可知,质点A、B、C的向心加速度之比为:8:4:3

故选C

阅读理解

阅读理解

     We drink tea every day. But more than three hundred years ago most of the people in

Europe(欧洲) did not know anything about tea. Some people heard about it, but very few

of them know what to do with it.

     There is a story about an English sailor(水手) who went to countries in the east, the west

and the south. He went to India and China. One day he came home and brought some tea as

a present for his mother. She told her friends about the present and asked them to a "tea party".

When her friends came to the "tea party", the old woman gave them brown tea-leaves. The old

woman's friends began to eat them. Of course, nobody liked the tea-leaves.

     At that time the sailor came in. He looked at the table and said, "Mother,what did you do

with the tea?"

     "I boiled it,as you said."

     "And what did you do with the water?"

     "I threw it away(扔掉),of course." answered the old woman.

     "Now you may throw away the leaves,too," said her son.

1. Most of the people in Europe __________ 300 years ago.

A. drank tea every day

B. knew everything about tea

C. knew nothing about tea

D. liked to eat the tea-leaves

2. One day the English sailor brought his mother some tea from __________.

A. countries in the west

B. countries in the south

C. India or China

D. a tea shop in England

3. The sailor's mother asked her friends to her house, because __________.

A. the sailor told her to

B she liked to show off(炫耀)

C. she wanted to ask her friends what to do with the tea

D. she wanted to taste the tea together with her friends

4. At the "tea party", __________.

A. all the woman's friends spoke highly of(高度评价) the tea

B .nobody knew what to do with the tea

C. the woman didn't eat the tea-leaves

D. the woman gave her friends each a glass of tea water

5. What mistake(错误) did the old woman make? She __________.

A. boiled the tea

B. did as the sailor said

C. threw away the water

D. didn't throw away the tea-leaves

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