问题 选择题

如图9-6-8为一单摆的振动示意图,下列说法中正确的是(   )

图9-6-8

A.在平衡位置处,摆球的功能和势能均达到最大值

B.在最大位移处,摆球的势能最大,动能最小

C.在振动过程中,摆球的总机械能随时间做周期性变化

D.在振动过程中,摆球的动能和势能都随时间做周期性变化

答案

BD

同一单摆振动过程中,其势能的大小是由上升的高度决定的,位移越大,势能越大;动能的大小是由速度决定的,速度越大,动能越大.单摆的总机械能等于势能和动能之和,且振动过程中动能与势能随时间做周期性的相互转化,总机械能守恒.在最大位移处,势能最大,动能为零;在平衡位置处,势能最小,动能最大;摆球远离平衡位置过程中,位移变大,势能增加,动能减少;反之,摆球向平衡位置运动过程中,动能增加,势能减少.所以选项A、C错误,B、D正确.

选择题
完形填空

Todd was working at his gas station(加油站) at night when he heard over the radio that a ___1___ in Long Island had been ___2___ by an armed man who had killed the night guard and got away with $150,000. “One hundred and fifty thousand,” Todd whistled. Here’s a fellow who just ___3___ into a bank and helps himself ___4___ so much money. Todd thought of the ___5___ with which he managed to get the amount of money he ___6___ to start his gas station. So many papers to ___7___. So much money to pay back.

The news ___8___ twenty minutes later. The gunman had ___9___ a car for a ride, and then ___10___ out the driver. He was possibly ___11___ the Southern State Parkway in a white Ford. License plate(车牌) number LJR 1939. The ___12___ of the announcer continued: “__13___ out for white cars. Don’t pick up ___14___, and all you folks in gas stations better not do ___15___ to a white Ford car.”

Todd stood up and ___16___ to see out into the cold night. It was dark but Todd ___17___ the Southern State Parkway was out there. Just ___18___, Todd saw the headlights coming at him and a car pulled in for ___19___. There it was, a white Ford. He saw the ___20___, LJR 1939.

“What should I do?” Todd had to make a quick ___21___.

“Yes, sir?” Todd ___22___ while making up his mind for sure.

“__23__ her up,” the man said sounding like any other ___24___.

When the tank(油箱) was full, Todd quickly turned round and pointed a gun at the man.

“Hands up ___25___ get out.”

1. A. store      B. bank   C. station D. house

2. A. searched for   B. held up      C. taken over  D. broken into

3. A. walks     B. looks  C. marches     D. drives

4. A. for  B. by      C. to       D. of

5. A. satisfaction    B. difficulty    C. disappointment  D. spirit

6. A. saved     B. made  C. offered       D. needed

7. A. collect    B. prove  C. sign    D. write

8. A. continued      B. lasted  C. spread D. arrived

9. A. bought   B. borrowed   C. stolen  D. stopped

10. A. sent      B. found  C. left     D. pushed

11. A. calling from       B. fleeing from      C. heading for D. looking for

12. A. news    B. warning     C. advice D. voice

13. A. Look    B. Run    C. Call    D. Set

14. A. guests   B. strangers    C. prisoners    D. passengers

15. A. harm    B. favor  C. service       D. business

16. A. tried     B. decided      C. hoped D. happened

17. A. considered   B. knew  C. recognized  D. learnt

18. A. then     B. there   C. right   D. now

19. A. directions    B. repairs       C. gas     D. parking

20. A. mark    B. number      C. sign    D. name

21. A. decision       B. call     C. movement  D. remark

22. A. wondered    B. stopped      C. waited D. asked

23. A. Cover   B. Fill     C. Check D. Tie

24. A. visitor  B. robber C. driver D. rider

25. A. or B. and     C. but     D. to