问题 选择题

如图所示,站在汽车上的人用手推车的力为F,车向右运动,脚对车向后的静摩擦力为Ff,下列说法正确的是(  )

A.Ff对车不做功,F对车做正功

B.Ff对车做负功,F对车不做功

C.当车匀速前进时,F、Ff的总功一定为零

D.当车加速前进时,F、Ff的总功一定为零

答案

A、B、脚对车向后的静摩擦力为Ff,人用手向前推车的力为F,车向前运动,故F对车做正功,Ff对车做负功,故A错误,B错误;

C、对人受力分析,受重力、支持力、车对人向后的推力和向前的静摩擦力;当车匀速前进时,推力和静摩擦力平衡;

然后根据牛顿第三定律,手推车的力F和脚对车向后的静摩擦力Ff相等,由于车的位移一定,故F、Ff的功等大,一正一负,总功一定为零,故C正确;

D、人受重力、支持力、车对人向后的推力和向前的静摩擦力;车加速前进时,静摩擦力大于推力;

然后根据牛顿第三定律,脚对车向后的静摩擦力Ff大于手推车的力F,由于车的位移一定,故摩擦力的功大于推力的功,总功不为零,故D错误;

故选:C.

阅读理解

第三部分阅读理解(共20题,每小题2分,满分40分)

Recently, tests have proved that Beethoven died from lead poisoning. Bill Walsh directed the Beethoven Research Project. Mr. Walsh and his team examined pieces of bone belonging to Beethoven. They found a large amount of lead in the bone pieces. They said the lead levels were equal to those found in pieces of his hair in earlier studies of other scientists.

Genetic tests proved the bone pieces and his hair came from Beethoven. The research also examined bone fragments from someone else who lived during the same period. Both were from the top of the skull. The fragments from Beethoven had more lead than those from other person. Their study didn’t find measurable levels of cadmium(镉) or mercury(汞), which scientists used to consider were the causes of his health problems.

Beethoven was sick for much of his life. He experienced strong stomach pains, a sign of lead poisoning, and a change of personality when he was around 20. he also got angry easily, and suffered from depression and hearing loss. His health problems became worse as he grew older.

Walsh said the lead levels found in his skull suggested that the metal might have been present in his body for many years. He said although there had been recognized cases of deafness caused by lead poisoning, there was no strong evidence to suggest that lead poisoning was the cause of his deafness.

Beethoven visited many doctors to find a cure for his health problems, in a letter he wrote to a friend, he urged researchers to examine his body after he died so that other people would not have to suffer as he did.

61. If too much lead is in a person’s body, he will______.

A. suffer hearing loss             B. get angry easily 

C. suffer serious stomach pains     D. lose most of his hair

62. From the passage we can learn that_______.

A. this is the first research into the cause of Beethoven’s death

B. any amount of lead will do great harm to people’s health

C. more lead was found in Beethoven’s bones than in his hair

D. scientists used to think that his death was caused by other metals instead of lead

63. In which section of the newspaper might you read this report?

A. Health.   B. Technology.   C. Entertainment.   D. Culture.

64. What is the best title of the passage?

A. A surprising discovery about Beethoven   B. Beethoven died from lead poisoning

C. Lead--- a killer metal for human beings    D. New examination of Beethoven’ body  

单项选择题