问题 选择题

在水平面上,小猴拉着小滑块做匀速圆周运动,O点为圆心。能正确地表示小滑块受到的牵引力F及摩擦力Ff的图是

A.              B.                C.                D.

答案

答案:C

题目分析:滑块做匀速圆周运动,合力指向圆心,对其受力分析,重力和向上的支持力平衡,物体速度方向沿切线方向,滑动摩擦力与相对地面的运动方向相反,沿切线向后,拉力与摩擦力的合力提供向心力,指向圆心,故只有C正确;

故选C.

点评:对物体受力分析通常要结合物体的运动情况分析,如果不看运动情况,往往无法将物体的受力情况分析清楚.

填空题

阅读下面的短文,用括号中所给动词的适当形式填空,必要时可加助动词。

     You may know the song Happy Birthday very well. But do you know about its writer? It was written by an

American girl. And now she 1.             (become) a very rich woman. 

     When she was a child,she was poor. Once she 2.             (invite) to her friends' birthday party. She was

very pleased but sad because she had not enough money to buy presents for them. "The party 3.             (come)

soon. Now I have little money." Tears ran down her face.

     Later that night she was in bed, thinking about the presents when the door opened and in came her grandma,

"What 4.             (happen)?" her grandma asked. Hearing the girl's story, she said, "5.             (not worry), I

think I 6.             (help) you. How about singing a song together? Happy birthday to…" What a beautiful song!

They sang and sang. Suddenly she woke up. It was a dream! She decided to write it down at once and 7.          

(sing) it to her friends at the party.

     When she sang the song at the party the next day, her little friends were very happy. "How wonderfully she

8.            (sing)! We 9.             (not hear) such a song before. Thank you for 10.             (give) us the special

present." said one of them. And they learnt to sing it together. Later the girl became well known in America.

解答题