问题 问答题

如图,甲、乙两同学在直跑道上练习4×100m接力,他们在奔跑时具有相同的最大速度.乙从静止开始全力奔跑需跑出25m才能达到最大速度,这一过程可看作匀加速直线运动.现在,甲持棒以最大速度向乙匀速奔跑,乙在接力区待机全力奔出.若要求乙接棒时的速度达到最大速度的80%,则乙应在距离甲多远处起跑.

答案

设最大速度为v.从乙起跑到接棒的过程中,甲乙运动时间t相同.

对于乙,其运动为匀加速直线运动.设加速度为a,则:

a=

v2
2s
 其中:s=25 m

设乙从起跑到接棒奔跑的距离为s1,则:s1=

(0.8v)2
2a
=16 m

对于甲,其运动为匀速直线运动.设其在乙从起跑的接棒这段时间t内的位移为s2,则:t=

s1
0.8v
2
或t=
0.8v
a

故:s2=vt=40 m

所以,乙起跑时,离甲的距离为:△s=s2-s1=24 m.

答:乙应在距甲24m处开始起跑.

完形填空

We walked in so quietly that the nurse at the desk didn’t even lift her eyes from the book. Mum __36__ at a big chair by the door and I knew she wanted me to __37__ down. While I watched __38__ open in surprise, Mum __39__ her hat and coat and gave them to me to hold. She walked __40__ to the small room by the lift and took out a wet mop. She __41__ the mop past the desk and __42__ the nurse looked up, Mum nodded and said, “Very __43__ floors.”

“Yes, I’m __44__ they’ve finally decided to clean them,” the nurse answered. She looked at Mum __45__ and said, “But aren’t you working late?”

Mum just pushed harder, each swipe of the mop __46__ her farther and farther down the hall. I watched __47__ she was out of sight and the nurse had __48__ to writing in the big book.

After a long time Mum came back. Her eyes were __49__. She quickly put the mop back and took my __50__. As we turned to go out of the door, Mum bowed politely to the nurse and said, “__51__.”

Outside, Mum told me: “Dogmas is __52__. No fever.”

“You saw her, Mum?”

“Of course. I told her about the hospital __53__, and she will not expect us until tomorrow. Dad will stop __54__ as well. It’s a fine hospital. But such __55__! A mop is no good. You need a brush.”

36. A. stared           B. pointed               C. came                   D. aimed

37. A. sit                B. lie                      C. fall                     D. bend

38. A. eyes                     B. hands                C. mouth               D. arms

39. A. put on          B. took off              C. hung over          D. threw away

40. A. bravely          B. proudly               C. poorly               D. quietly

41. A. dragged               B. pulled                  C. pushed          D. rushed

42. A. as                 B. because               C. after                 D. as soon as

43. A. pretty           B. good                   C. cheap                D. dirty

44. A. delighted            B. surprised             C. anxious           D. disappointed

45. A. respectfully           B. pitifully               C. carefully           D. strangely

46. A. taking           B. fetching              C. forcing          D. advancing

47. A. when          B. while                  C. after                   D. until

48. A. turned back       B. turned round      C. turned down   D. turned into

49. A. nervous                B. sad                     C. crying               D. shining

50. A. clothes           B. hat                      C. hand             D. mop

51. A. Excuse me      B. I’m sorry            C. Good luck           D. Thank you

52. A. ill                 B. better               C. well              D. wrong

53. A. rules             B. rights                   C. advice               D. limits

54. A. visiting           B. worrying             C. smoking        D. working

55. A. a hospital         B. beds                    C. floors                D. nurses

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