问题 选择题

一个在水平桌面上滑行的木块,水平方向受到两个力的作用,且两个力的三要素都相同(物体可视为一个点),下列叙述正确的是(  )

A.木块的运动状态一定发生改变

B.木块的运动状态不发生改变,保持原来的速度做匀速直线运动

C.这两个力的合力可能等于零

D.这两个力的合力的方向与物体运动的方向相同

答案

A、两个力的三要素都相同,说明都朝向同一方向,那么,物体一定受到的是不平衡的力的作用,因此,运动状态一定发生改变,所以正确;

B、从A的分析中可知,物体受到的是不平衡力的作用,因此,不可能做匀速直线运动,所以错误;

C、木块受非平衡力的作用,其合力不为零,所以错误;

D、这两个力的合力的方向可能与物体运动的方向相同,也可能与物体运动的方向相反,比如,物体由于惯性向前运动,另有两个力阻碍其运动时,合力的方向可能与运动方向相反,所以错误.

故选A.

阅读理解

阅读理解.

     The summer vacation is over. It's true that time always flies fast. During the vacation, the weather was

hot and I could not do much work, but I lived happily.

     As the afternoon was hot, I did my work in the morning. I used to get up at 6: 30 and take a walk in

the garden for half an hour. After breakfast, I began reading English and Chinese and did some exercises

in maths. Those took me three hours or more I worked quite hard and made good progress.

     I spent the afternoon outside. I went to swim and it was funny. I would not go home until it was about

five or six o'clock. Sometimes a friend of mine would come to see me and we would spend some hours

listening to music.

     In this way I spent my vacation happily. And I not only studied well but also became a good swimmer.

Now I am in good health and high spirits.

1. What does the text mainly tell us?                                  

   A. How the writer, spent his summer holidays.                      

   B. What the weather was like in that summer.                        

   C. When the writer got up in the morning.                          

   D. Where the writer took a walk.                                    

2. How long did it take the writer to do his homework?                

   A. Half an hour.                                                    

   B. Three hours or more.                                            

   C. Only one hour.                                                  

   D. Two hours and a half.                                            

3. The writer spent most of the afternoon            .                            

   A. listening to music                                              

   B. visiting his friends                                            

   C. walking in the garden                                            

   D. swimming                                                        

4. The writer had very good summer holidays because he            .              

   A. worked very hard and made good progress                          

   B. learned to swim and did his work well                            

   C. liked swimming better than studying                              

   D. got up early and went home late                                  

5. Which of the following is NOT true?                                

   A. The writer made progress in his lessons.                        

   B. The writer took a walk for half an hour before breakfast.        

   C. The writer began studying as soon as he got up.                  

   D. The writer spent more time swimming than listening to music.    

单项选择题