问题 计算题

表演“顶杆”杂技时,一个人站在地上(称为“底人”)肩上扛一长L=6 m,质量m1=15 kg的竹竿,一质量m2=45 kg的演员(可当质点处理)在竿顶从静止开始先匀加速再匀减速下滑,下滑时加速度大小相等,下滑的总时间为t=3 s,演员恰从杆顶滑至杆底部.求:

(1)演员下滑过程中的最大速度;

(2)演员在减速下滑过程中竹竿对“底人”的压力.

答案

解:(1)演员下滑到加速阶段结束时速度最大,设为v,则

(2)减速阶段对演员有:Ff-m2g=m2a

得Ff=m2(g+a)=570 N

根据牛顿第二定律得竹竿对“底人”压力为FNFf+m1g,代入数据FN=720 N

单项选择题 A1/A2型题
阅读理解

They’re WILD animals

By Ernst-Ulrich Franzen

March 11, 2010 (3) Comments

The story about the woman who lost some fingers while feeding a bear at a zoo in Manitowoc, after she ignored warnings and barriers(栅栏), reminded me of the story I heard about a couple who put their baby on the back of a wild horse in South Dakota to get a really cute picture. We all do silly things at times — no one is immune — but treating wild animals as lovely pets has to fall into a special category. Teddy bears and Disney movies aren’t actually representative of real bears.

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1. TosaLeft - Mar 11, 2010 10:46AM

Don’t you think that maybe, just maybe some alcohol was involved?

2. tk421 - Mar 11, 2010 11:09 AM

It was already approved that alcohol was involved. Stories that begin with a drunk person saying “Hey, I got an idea, watch this!” rarely end well.

3. Tristan Kloss - Mar 11, 2010 11:41 AM

Alcohol certainly isn’t involved when people decide to keep “pets” like chimpanzees, baby tigers, etc. Stupidity, definitely. Dogs are pets because of thousands of years of domestication. Even farm animals, which have been kept by humans for thousands of years as well, aren’t let in the house. So why keep animals that treat human contact with, at best, indifference(冷淡、不在乎) and, at worst, violence?

60.In Ernst-Ulrich Franzen’s opinion, the woman lost her fingers because ________.

A. the zoo keepers didn’t warn her of the danger       

B. she didn’t know the bear was a wild animal

C. she was somehow influenced by cartoon characters

D. she climbed over the barriers and angered the bear

61.TosaLeft thinks the wounded woman ________.

A. may have been drunk                                          B. may be a little stupid

C. was addicted to wine                                           D. fed wine to the bear

62.tk421 means a drunk person ________.

A. should be forbidden to enter the zoo                     B. usually gets himself into trouble

C. is often fond of making up stories                         D. usually likes to show himself off

63.What does Tristan Kloss think of people treating wild animals as pets?

A. Kind.                       B. Illegal.                                   C. Loving.                           D. Stupid.