问题 选择题

下列说法中,正确的是(  )

A.物体的动能不变,则物体所受的外力的合力必定为零

B.物体的动能变化,则物体所受的外力的合力必定不为零

C.物体的速度变化,则物体的动能必定发生变化

D.物体所受的合外力不为零,物体的动能必定发生变化

答案

A、如果动能不变说明合力对物体做的功为零,可能是合力与速度方向垂直,但是合力不一定为零,比如匀速圆周运动,故A错误.

B、物体的动能变化,则合力做功一定不为零,则合力必定不为零,故B正确;

C、物体的速度变化,可能只是速度的方向发生变化而速度大小不变,则动能不变,故C错误;

D、物体所受的合外力不为零,但若合外力与速度方向垂直,则合外力做功为零,根据动能定理则物体的动能不变,故D错误.

故选:B.

计算题
句型转换

句型转换。

1. English is not so interesting as Chinese. (改为同义句)

    English is _______ interesting _______Chinese.

2. Antonio doesn't do well in Chinese.(改为同义句)

    Antonio _______ _______ _______ Chinese.

3. Why not join an English language club?(改为同义句)

    Why _______ _______ _______ an English club?

4. Can you tell me how I use the computer?(改为同义句)

    Can you tell me _______ _______ use the computer?

5. We have P.E. lessons on Thursday and Friday.(改为同义句)

    We have P.E. lessons _______ a week.

6. If you want to improve English, you should practice writing a lot.(同义句)

    _______ _______writing practice can help you improve your English.

7. It's very easy for me to find his office.(改为同义句)

    I had _______ _______ _______ his office.

8. It wasn't easy for me to understand the teacher in class.(改为同义句)

    It _______ _______ for me to understand the teacher in class.

9. It makes my listening skills better. (改为同义句)

    It _______my listening skills.

10. The teacher isn't pleased with him.(改为同义句)

    The teacher _______ _______with him.

11. My watch doesn't work well.(改为同义句)

     There is _______ ________ ________ my watch.

12. Shall we meet at half past two? (改为同义句)

     What ________ _________ at half past two?

13. It's bad for your eyes to read in the sun. (改为同义句)

     ________ in the sun _______ bad for your eyes.

14. How much did the new mobile phone cost? (改为同义句)

      How much did you _______ _______the new mobile phone?

15. The girl is so young that she cant' go to school.(改为同义句)

      The girl is _______ young _______ go to school.