问题 计算题

用如图8-6-6所示装置验证动量守恒定律,质量为mA的钢球A用细线悬挂于O点,质量为mB的钢球B放在离地面高度为H的小支柱N上,O点到A球球心的距离为l,使悬线在A球释放前绷紧,且线与竖直线的夹角为α,A球释放后摆动到最低点(初始位置是水平的)时恰好与B球正碰,碰撞后,A球把轻质指示针OC推移到与竖直方向夹角β处,B球落在地面上,地面上铺一张盖有复写纸的白纸D,保持α角不变,多次重复上述实验,白纸上记录到多个B球的落点.

图8-6-6

(1)图中s应是B球初始位置到_________的水平距离.

(2)为了验证两球碰撞过程动量守恒,应测得_________等物理量.

(3)用测得的物理量表示碰撞前后A球、B球的动量:pA=_____,pA′=_________,pB=_______,pB′=_________.

答案

(1)落地点

(2)mA、mB、α、β、H、s、L

(3)mAmA  0  mBs 

图中s应是B球初始位置到落地点的水平距离.

碰前A球到最低点时的速度满足mvA2=mgl(1-cosα)

pA=mAvA=mA

B球:pB=0

碰后:mvA2=mgl(1-cosβ)

pA′=mAvA′=mA

vB′=,pB′=mBs.

阅读理解

阅读理解。

Dear Malia and Sasha,

      I know that you've both had a lot of fun these two years. You have been to many picnics and had all

kinds of junk food your mother and I probably shouldn't have let you eat. But I also know that it hasn't

always been easy for you and Mom. So now I want to tell you a little more about why I decided to take our

family on this journey.

     When I was a young man, I thought life was all about me-about how I'd make my way in the world,

become successful, and get the things I want. But then the two of you came into my world with those

smiles and suddenly, all my big plans didn't seem so important any more. I soon found that the greatest

happiness in my life was that you were so happy. And 1 realized that my own life wouldn't mean much if

I couldn't make you two happy. In the end, girls, I ran for (竞选) President: because of what I want for you

and for every child in the US.

     I want all our children to go to school so that they can learn and grow. I want them to have the chance

to go to college-even though their parents don't have enough money. And I want them to get good jobs.

     I want us to see new technologies and inventions that improve our lives and make our earth cleaner and

safer. And I want us to see no difference of different colors and areas because we are all equal (平等的).    

     I hope both of you will start to give others the chances you've had. You know, this country has given

our family so much, so you have to, and really should, give something back. When you are doing something

larger than yourself, you will realize how helpful you are.

     These are the things I want for you-to grow up in a world in which you can make your dreams come

true. And I want every child to have the same chances to learn and dream and grow and get what you girls

have. That's why I've taken our family on this great adventure.

      I am so proud of you. I love you more than you can ever know. And I am thankful every day because

you behave so well when we get ready to start our new life together in the White House.

                                                                                                                                      Love, Dad

1. From the letter we can see that the father is President of ________.

A. the US

B. the UK

C. Australia

D. Canada

2. According to the second paragraphy, the greatest happiness in the father's life was that ________.

A. he could be President

B. he was very successful

C. his two children were so happy

D. he could get all the things he wanted

3. The underlined word"colors" in paragraph 4 most probably means ________

A. people

B. lifestyles

C. inventions

D. cities

4. What can we learn from the last paragraph?

A.The children don't behave well enough.

B.The children know their father's love well.

C.The children are very proud of their father.

D. The family will live in the White House soon.

5. Which of the following can be the best title of this passage?

A. How to be a president

B. What I want for you

C. A journey to the White House

D. We love you, Dad!

单项选择题