问题 计算题

汽车发动机的功率为60kw其总质量为5t.在水平路面上行驶时,所受阻力恒为5.0 ×103N.试求(1)汽车所能达到的最大速度

(2)若汽车以0.5m/s,的加速度由静止开始与加速运动 这一过程能维持多长间?

答案

12m/s、16s

:(1)当汽车达最大速度时,由于汽车做匀速运动,牵引力等于阻力,其a=0,合外力为零,即有P0=Fvm=Ffvm

则最大速度为vm==12m/s

(2)由牛顿第二定律得:F-Ff=ma

则匀加速的牵引力为:F=Ff+ma=(5×103+5×103×0.5)N=7.5×103N

由P0=Fv得匀加速过程的最大速度为:v==8m/s由v=at可得:

这一过程维持的时间为:t==16s

阅读理解

阅读理解。

     Robert was born in a small town in England. His father has a farm and can supply him enough money to

finish university where he is studying law. The young man studies hard and hopes to be a famous lawyer.

     Last Monday their term was over. Robert said to his father on the telephone that he was going to travel in

a small country during his summer holiday. His father agreed to his plan and posted some money to him. So

he started four days ago. Their plane landed at the only airport of the country safely. He took a taxi and got to

a hotel in the center of the capital. He felt hungry and tired. So he had a dinner and then a good sleep. This

morning he was all right and was going to visit some places of interest. He met an English visitor while he was

having breakfast and the man told him to be careful of thieves.

     Robert thanked the man and left. But he thought he was smart and strong and he decided to have a try. He

brought a piece of paper and wrote on it, "A pig has stolen my wallet!" Then he put the note into his empty

wallet. After that he put the wallet into a pocket and set off. He did all carefully and hoped to find out who

would steal it.

     "Everything went well, "Robert thought to himself after he had returned to the hotel. He brought out his

wallet and put his money into it again. He had a look at the note. To his surprise, it was rewritten. It said,

"Your uncle has touched your wallet!"

1. Robert can study in the university because            . [ ]

A. he wants to be a lawyer

B. his father is a rich farmer

C. he's smart and strong

D. he keeps his wallet well

2. Robert went to the small country to            . [ ]

A. make fun of the thieves

B. catch some thieves

C. visit the places of interest

D. have a nice meal and a good sleep

3. The English visitor thought            . [ ]

A. there were a lot of thieves in the city

B. Robert could deal with the thieves

C. it was dangerous to travel in the city

D. Robert had to take good care of himself

单项选择题