问题 选择题

铁路在弯道处的内外轨道高度是不同的,如图2所示,已知内外轨道平面对水平面倾角为θ,弯道处的圆弧半径为R,则质量为m的火车在该弯道处转弯时

A.若火车行驶速度等于,则内外轨道均不受挤压

B.若火车行驶速度太小,内轨对内侧车轮轮缘有挤压

C.若火车行驶速度太小,外轨对外侧车轮轮缘有挤压

D.若火车行驶速度等于,这时铁轨对火车的支持力等于

答案

ABD

火车的重力和轨道对火车的支持力的合力恰好等于需要的向心力时,此时火车的速度正好是,当火车火车转弯的速度小于时,需要的向心力减小,而重力与支持力的合力不变,所以合力大于了需要的向心力,内轨就要对火车产生一个向外的力来抵消多余的力,所以此时内轨对内侧车轮轮缘有挤压,A正确,B正确.

火车的速度是,可以把支持力力分解为水平和竖直向上两个分力,由于竖直向上的分力和重力平衡,且支持力为斜边,所以火车的支持力等于,D正确

故选ABD.

阅读理解

For a commuter rushing to catch a train, a minute can mean the difference between dinner with the family and leftovers (剩饭) in the microwave. What most passengers don’t realize is that their minute is already there.

Every commuter train that departs from New York City — about 900 a day — leaves a minute later than scheduled. If the timetable says 8:14, the train will actually leave at 8:15. In other words, if you think you have only a minute to get that train — well, relax. You have two.

The courtesy (礼貌) minute, in place for decades and published only in private timetables for employees, is meant as a grace period(宽限期)for those who need the extra time to get off the platform and onto the train.

“If everyone knows they get an extra minute, they’re going to waste time doing unimportant things,” explained Marjorie Anders, a spokeswoman for the Metro-North Railroad. Told of this article, Ms. Anders laughed. “Dont blow our cover!” she said.

Entirely hidden from the riding public, the secret minute is an odd departure from the railroad culture of down-to-the-second accuracy. The railroad industry helped invent the concept of standard time, and time zones were established in the United States in the 1880s, 35 years before they were written into law. And most commuters know their train by the precise minute it departs. The trains quickly make up the minute: at all other stops, the public timetable prevails. The phantom minute does not exist at commuter railroads in Chicago, Los Angeles, Philadelphia, or San Francisco. But in New York, the secret minute dates back decades.

“That’s been done forever, from my knowledge,” said Jack Swanberg, 70, an unofficial historian of Metro-North. “I was the trainmaster starting in 1970 and I’m sure it’s been the case since 1870 for all I know.”

小题1:The courtesy minute was hidden from the public to _____.

A.prevent the passengers’ idleness

B.help invent the concept of standard time

C.show the railroad company’s consideration

D.follow the ancient tradition of New York City小题2: The underlined part “Don’t blow our cover” in Para. 4 probably means “_____”.

A.Don’t publish the timetable

B.Don’t blame commuter trains

C.Don’t make it known by the public

D.Don’t forget our chances of catching trains小题3:What can we conclude from the passage?

A.The courtesy minute exists in many cities in the US.

B.One minute means a lot for most of the commuters.

C.The courtesy minute has been in place for about ten years.

D.Most railroad staff in New York have no idea of the courtesy minute.小题4: The passage mainly talks about _____.

A.the railroads in the US

B.the secret New York minute

C.the mistake of the railroad industry

D.the history of New York commuter trains

单项选择题