问题 问答题

一辆汽车在平直的路面上以恒定功率由静止开始行驶,设所受阻力大小不变,其牵引力F与速度V的关系如图所示,加速过程在图中B点结束,所用的时间t=10s,经历的路程s=60m.

求:(1)汽车所受阻力的大小;(2)汽车的质量.

答案

(1)加速过程在B点结束,即此后汽车沿平直路面作匀速运动,由平衡条件和图象信息可得

    FB-f=0  ①

    FD=104N  ②

由①②得阻力 f=104N

(2)由图象信息得:汽车的功率 P=Fv=104×10W=105W

汽车加速运动过程,牵引力做功为 W=pt  

根据动能定理可得 Pt-Fs=

1
2
mv2

则得:m=

2(Pt-fs)
v2

代入数据得:m=8000 kg

答:

(1)汽车所受阻力的大小=104N;

(2)汽车的质量是8000kg.

单项选择题
阅读理解

Reading comprehension

     Last week I visited one of my old neighborhoods in Washington D.C. I had not been there for twenty

years and as I walked along the street, my mind was flooded by memories of the past. I saw the old

apartment building where I had lived and the playground where I had played. As I viewed these once

familiar surroundings, images of myself as a child there came to mind. However, what I saw and what I

remembered were not the same. I sadly realized that the best memories are those left untouched.

     My old apartment building, as I remember, was bright and alive. It was more than just a place to live.

It was a movie house, a space station, or whatever my young mind could imagine. I would steal away

with my friends and play in the basement. This was always exciting because it was so cool and dark, and

there were so many things there to hide among. There was a small river in the back of the building. We

would go there to lie in the shade of trees and enjoy ourselves.

     However, what I saw was completely different. The apartment building was now in disrepair. What

was once more than a place to live looked hardly worth living in. The windows were all broken. The

once clean walls were covered with dirty marks. The river was hardly recognizable. The water was

polluted and the trees and flowers were all dead. The once sweet-smelling river now smelled terrible. It

was really heartbreaking to see all this.

     I do not regret having seen my old neighborhood. However, I do not think my innocent childhood

memories can ever be the same. I suppose it is true when they say, "You can never go home again."

1. In the first two paragraphs of the passage the writer tries to tell us that ________.

A. he had lived in Washington D.C. for twenty years

B. he was very imaginative when he was a child

C. he had had pleasant memories of his old neighborhood

D. he had come from a very poor neighborhood

2. How did the writer feel when he visited his old neighborhood in Washington D.C.?

A. Greatly surprised.

B. Very regretful.

C. Quite excited.

D. Very sad.

3. On his visit to his hometown, the writer found the old apartment building ________.

A. seemed unfit to live in

B. had been repaired

C. could hardly be recognized

D. looked dirty and smelt horrible

4. "You can never go home again" in the last sentence of this passage means that ________.

A. one should never revisit his hometown after many years of absence

B. one will never find his hometown the same as it was in his childhood memory

C. a visit to one's hometown will bring back one's pleasant memories of home

D. a visit to one's hometown will bring back many sad memories