问题 选择题

用如图所示的滑轮组将重10N的物体匀速提升0.1m,所用时间为2s,作用在绳子末端的拉力F为6N(不计绳重和绳与滑轮间的摩擦),下列计算结果正确的是(  )

A.所做的有用功为1.2J

B.动滑轮自重0.2N

C.拉力F做功的功率为0.3W

D.该滑轮组的机械效率为83.3%

答案

答案:D

题目分析:A、∵用滑轮组提升物体,对物体做的功是有用功.

∴所做的有用功,故A错;

B、不计绳重和绳与滑轮间的摩擦时,,则:动滑轮的重力,故B错;

C、用滑轮组提升物体,拉力F做的功是总功,则拉力F做功的功率:

,故C错;

D、∵该滑轮组的机械效率,故D正确.

故选D.

点评:本题考查了使用滑轮组拉力的计算、机械效率的计算,根据题图确定n的大小(直接从动滑轮上引出的绳子股数)是本题的突破口,利用好不计绳重和摩擦时拉力和物重的关系是本题的关键.

完形填空

第三节:完形填空(共20小题;每小题1.5分,满分30分)

阅读下面短文,从短文后所给各题的四个选项(A、B、C和D)中,选出可以填入空白处的最佳选项,并在答题卡上将该项涂黑。

I hired a plumber(水管工) to help me restore an old farmhouse, and he just finished a rough first day on the job: a  41 tire made him lose an hour of work, his electric drill quit and his ancient one ton truck refused to  42  .

   43  I drove him home, he sat in stony silence.    44  arriving, he invited me in to meet his   45 . As we walked toward the front door, he paused briefly at a small   46 , touching the tips of the branches with both   47 .

When   48 the door he passed through an amazing transformation. His brown face was in all smiles and he hugged his two small children and gave his   49 a kiss.  50 he walked me to the car. We passed the tree and my   51 got the better of me. I asked him about what I had   52 him do earlier.

"Oh, that's my   53 tree," he replied. "I know I can't help having troubles on the job, but one thing's   54 :those troubles don't   55 the house with my wife and the children. So I just hang them up on the tree    56  when I come home and ask God to    57 them. Then in the morning I   58 

them up again." "And the   59 thing is," he smiled, "when I come out in the morning to pick them up, there aren't   60 as many as I remember hanging up the night before."

41. A. smooth                  B. broken                  C. failed                     D. flat

42. A. begin                     B. start                       C. set                         D. launch

43. A. While                    B. During                   C. As soon as              D. Though

44. A. In                         B. For                        C. On                         D. At

45. A. friends                   B. parents                   C. family                    D. home

46. A. room                     B. tree                       C. flower                    D. wood

47. A. arms                      B. hands                     C. fingers                   D. feet

48. A. opening                 B. closing                   C. knocking                D. covering

49. A. wife                      B. daughter                C. son                        D. baby

50. A. Meanwhile             B. Afterwards             C. Previously              D. Consequently

51. A. satisfaction             B. responsibility          C. occupation              D. curiosity

52. A. heard                     B. made                     C. seen                       D. let

53. A. comfort                 B. pain                       C. suffering                D. trouble

54. A. at length                B. for sure                  C. at risk                   D. straight away

55. A. belong in               B. hold back               C. fit in                      D. bring to

56. A. one day                 B. every morning        C. one night                D. every night

57. A. take note of            B. take care of            C. look for                 D. focus on

58. A. pick                      B. take                       C. bring                     D. choose

59. A. flexible                  B. funny                    C. casual                    D. humorous

60. A. almost                   B. roughly                  C. nearly                    D. approximately

判断题