问题 选择题

如图所示,粗糙的水平面上放有一个截面为半圆的柱状物体A,A与竖直挡板间放有一光滑圆球B,整个装置处于静止状态.现将挡板水平向右缓慢平移,A始终保持静止.则在B着地前的过程中(  )

A.挡板对B的弹力减小

B.地面对A的摩擦力增大

C.A对B的弹力减小

D.地面对A的弹力增大

答案

先对B受力分析,受重力、A对B的支持力和挡板对B的支持力,如图:

根据共点力平衡条件,有:

N1=

mg
cosθ

N2=mgtanθ

再对AB整体受力分析,受重力、地面支持力、挡板对其向左的支持力和地面对其向右的静摩擦力,如图:

根据共点力平衡条件,有

f=N2

N=(M+m)g

f=mgtanθ

挡板保持竖直且缓慢地向右移动过程中,角θ不断变大,故f变大,N不变,N1变大,N2变大;

故选:B.

阅读理解

I was doing some Christmas shopping in a toy store and decided to look at Barbie dolls for my nieces. A nicely dressed little girl was excitedly looking through the Barbie dolls as well. As she was looking, a little boy came to the Pokemon toys. He was dressed neatly, but his clothes were obviously old. He was with his father as well, and kept picking up the Pokemon video toys. Each time he picked one up and looked at his father, his father shook his head and said, “No.”

The little girl had chosen her Barbie. However, she stopped and was watching them. Rather dejectedly, the boy had to give up the Pokemon toys and choose something else. The little girl put her Barbie back on the shelf, and ran over to the Pokemon toys. She excitedly picked up one and raced towards the checkout. I picked up my purchases and got in line behind them. Then, much to the little girl’s delight, the little boy and his father got in line behind me. After the toy was paid for and bagged, the little girl handed it back to the cashier (收银员) and whispered something in her ear. The casher smiled and put the package under the counter.

I paid for my purchases and was rearranging things when the little boy came up to the cashier. The cashier checked his purchases and said, “Congratulations, you are my hundredth customer today, and you win a prize!” With that, she handed the little boy the Pokemon toys, and he could only stare in surprise. It was exactly what he had wanted!

The little girl and her father had been standing at the doorway during all of this. Then they walked out. As I walked back to my car, I heard the father ask his daughter why she had done that. I’ll never forget what she said to him. “Daddy, didn’t Grandparents want me to buy something that would make me happy?” He said, “Of course they did, honey.” To which the little girl replied, “Well, I just did!”

I feel very shocked to have witnessed the true spirit of Christmas in that toy store, in the form of a little girl who understands more about the meaning of this festival than most adults I know.

小题1:The underlined word “dejectedly” in the second paragraph probably means _____.

A.delightedly

B.disappointedly

C.excitedly

D.unexpectedly小题2:How did the little boy get the Pokemon toys?

A.His father bought him the Poknemon toys.

B.Fortunately, he was the hundredth customer and won the Poknemon toys.

C.The cashier felt sympathy for the little boy and gave him the Poknemon toys.

D.The little girl bought the Poknemon toys for him.小题3:What did the little girl mean when saying “Daddy, didn’t Grandparents want me to buy something that would make me happy?”

A.Her Grandparents wanted her to be happy.

B.Making the little boy happy made her happy.

C.The Poknemon toys made her happy.

D.Her Grandparents hoped that she could help others.小题4:Which of the following statements is NOT true according to the passage?

A.the story happened before Christmas.

B.The little boy had not enough money to buy the Poknemon toys.

C.Although the little girl liked the Poknemon toys very much, she gave them to the boy.

D.The author was deeply moved by the behavior of the little girl.

单项选择题