问题 选择题

如图所示,质量为m的木块,从半径为r的竖直圆轨道上的A点匀速滑向B点,则在这个过程中(  )

A.木块所受向心力方向不一定指向圆心

B.木块所受合外力方向始终指向圆心

C.木块对轨道的压力一直不变

D.木块的向心力逐渐增大

答案

AB、物体从A点滑到B点做匀速圆周运动,合外力指向圆周,提供向心力,故A错误,B错误;

CD、物体从A点滑到B点做匀速圆周运动,所需要的向心力不变,由重力和支持力的合力提供向心力,重力和支持力的夹角改变,所以压力改变,故CD错误.

故选:B.

阅读理解

阅读理解。

To: Jenny@126.com

From: Liming@yahoo.com.cn

Subject: Autumn!

Date:14/09/ 8:00 p.m.

Hello, Jenny!

     Today is the Mid-Autumn Festival. We will watch the moon tonight, and look for Chang'e and Wu

Gang. We usually eat mooncakes. They are round, like the moon.

     Our National Day is also in autumn, on October 1st. My mother and father are usually free for seven

days from October 1st to 7th! We will do many things together. I like going to some National Day

celebrations.

     But the best thing in autumn is my birthday. It's in October. I will be fifteen years old.

     Best wishes,

                                                                                            Yours,

                                                                                                 Li Ming

To: Liming@yahoo.com.cn

From: Jenny@126.com

Subject: _____!

Date:14/09/ 9:12 p.m.

Dear Li Ming,

     Thank you for your e-mail! I love autumn too. In Canada, we have an autumn holiday called Thanks-

giving Day. Family members always celebrate it together. I will go to my grandmother and grandfather's

house. My uncles, aunts and their children will be there too.

     Canadian Thanksgiving Day is in early October. In the USA, they celebrate Thanksgiving Day in late

November.

     Have a good time on your birthday, Li Ming! On my birthday, I have a cake with candles. Everyone

at the party sings the song "Happy Birthday". Then I blow out the candles! Will you have a cake for your

birthday?

                                                                                               Your friend,

                                                                                                            Jenny

1. Li Ming's birthday is _____.

A. on Sept. 14th

B. in September

C. before Oct. 1st

D. in October

2. Americans celebrate Thanksgiving Day _____.

A. in early October

B. in early November

C. in late October

D. in late November

3. Canadians usually _____ on Thanksgiving Day.

A. eat cakes

B. stay together

C. watch the moon

D. sing a song

4. The subject of Jenny's e-mail is "_____".

A. Your birthday

B. My birthday

C. Thanksgiving Day

D. National Day

5. Li Ming and Jenny both use the _____ when they write to each other.

A. pen

B. paper

C. computer

D. telephone

单项选择题