问题 计算题

水平地面上有一个重为4 000 N的箱子,箱子的底面积为4 m2。现用起重机把箱子吊到3 m高的位置,所用时间为10 s,起重臂下的钢丝绳是绕在一个动滑轮上的,如图所示。钢丝绳上的拉力为2 500 N。试求:

(1)起重机起吊前,箱子对地面的压强是多少?

(2)起重机起吊该箱子的机械效率是多少?

(3)钢丝绳上拉力的功率是多少?

答案

(1)起重机起吊前,箱子对地面的压强是1 000 Pa。

(2)起重机起吊该箱子的机械效率是80 %。

(3)钢丝绳上拉力的功率是1 500 W。

题目分析:已知:G=4 000 N  S=4 m2  h=3 m  n=2  t="10" s  F ="2" 500 N   (1分)

求:(1)p(2)η(3)P

解:(1)p=F/S=G/S=4 000 N/4 m2=1 000 Pa                           (1分)

(2)W=F1h=Gh=4 000 N×3 m=12 000 J

W=F2s=F2·nh=2 500 N×2×3 m=15 000 J                          (1分)

η=W/W=12 000 J/15 000 J=80%                                   (1分)

(3)P=W/t=15 000 J/10 s=1500 W                                  (1分)

点评:本题考查压强、有用功、总功、机械效率、功率的计算,关键是公式及其变形的灵活运用,还要知道在水平面上压力等于物体自身的重力.

单项选择题 案例分析题
完形填空

One of my fondest memories as a child is going by the river and sitting idly(闲散地)on the bank. There I would enjoy the peace and___1___, watch the water ___2__downstream, and listen to the chirps(喳喳声)of the birds and the rustling (沙沙声)of the leaves in the trees. I would  also___3___the bamboo trees___4___under pressure from the wind and watch them ___5____gracefully to their upright or ___6___ position after the wind had___7____. When I think about the bamboo tree’s ___8____to bounce back or return to it’s original position, the word resilience(弹性)__ 9___to mind. When used in ___10_____ a person , this word means the ability to readily _11__from shock, depression or any other situation___12____stetches the limits of a person’s emotions. Have you ever felt like you are about to shout? Have you ever felt like you are at your __13__point?__14__, you have survived the experience to live to talk about it. During the experience you probably felt a mix of __15____that threatened your health. You felt emotionally tired,___16_____ exhausted and you most likely endured unpleasant physical symptoms.

Life is a __17__ of good times and bad times, happy moments and unhappy moments. The next time you are __18___one of those bad times or unhappy moments that take you __19____to your breaking point, bend but don’t break. Try your best not to let the situation get the best of you. A measure of hope will__20__ you through the unpleasant test.

1.A. quiet  B. beautiful     C. noisy     D. happy

2.A. flow    B. rush     C. go     D. come

3. A. see     B. notice    C. watch   D. look at

4.A. bend    B. swing    C. break   D. tremble

5.A. return   B. recover  C. give   D. fly

6.A. straight  B. original  C. quick  D. necessary

7.A. died from  B. died out C. died down  D. died off

8.A. ability   B. power C. energy  D. strength

9.A. strikes  B. goes   C. appears  D. comes

10.A. preference to  B. conference to C. reference to D. appearance to

11.A. recover  B. calm  C. comfort  D. sink

12.A. where   B. that   C. when  D. in which

13.A. high   B. low   C. breaking   D. returning

14.A.Thankfully B. Hopefully  C. Happily  D. Surprisingly

15.A. senses  B. emotions  C. thoughts  D. minds

16.A. physically  B. mentally  C. patiently  D. fully

17.A. way     B. mixture   C. condition    D. feeling

18.A. experiencing  B. making   C. living  D. meeting

19.A. away  B. far  C. close   D. fast

20.A. bring  B. take  C. make  D. set