问题 计算题

某质量为1100kg的小汽车在平直路面试车,当达到20m/s的速度时关闭发动机,经过50s停下来。(g=10m/s2)求:

(1)小汽车关闭发动机通过的位移大小;

(2)小汽车受到阻力的大小;

(3)根据题目的情景,提出一个粗略估测小汽车发动机牵引力大小的方法。

答案

解:(1)关闭发动机后汽车的加速度为:=,负号表示加速度的方向与小汽车的速度方向相反,小汽车作减速运动

小汽车关闭发动机通过的位移大小:s=

(2)关闭发动机后,汽车只受阻力作用,f=m=1100×(-0.4)N=-440N

(3)方法1:根据平衡条件,当小汽车匀速前进时,牵引力F等于阻力f

方法2:小汽车从静止开始作匀加速运动,测量小汽车达到20m/s的加速时间,可计算出小汽车的加速度,根据F-f=ma可得牵引力F=ma+f

阅读理解

阅读理解。

     Among rich countries, people in the United States work the longest hours. They work much longer than

in Europe. This difference is quite surprising because productivity per hour worked is the same in the United

States as it is in France, Spain and Germany, and it is growing at a similar speed.

     In most countries and at most times in history, as people have become richer they have chosen to work

less. In other words they have decided to "spend" a part of their extra income on a fuller personal life. Over

the last fifty years Europeans have continued this pattern, and hours of work have fallen sharply. But not in

the United States. We do not fully know why this is. One reason may be more satisfying work, or less

satisfying personal lives.

     Longer hours do of course increase the GDP (国内生产总值). So the United States has produced more

per worker than, say, France. The United States also has more of its people at work, while in France many

more mothers and older workers have decided to stay at home. The overall result is that American GDP per

head is 40% higher than in France, even though productivity per hour worked is the same.

     It is not clear which of the two situations is better. As we have seen, work has to be compared with other

values like family life, which often get lost in interest. It is too early to explain the different trends (趋势) in

happiness over time in different countries. But it is a disappointing idea that in the United States happiness has

made no progress since 1975, while it has risen in Europe. Could this have anything to do with trends in the

work-life balance (平衡)?

1. From the text we know that the author _____.

A. believes that longer working hours is better

B. prefers shorter working hours to longer ones

C. says nothing certain about which pattern is better

D. thinks neither of the patterns is good

2. Which of the following countries has more of its people at work?

A. Spain

B. France

C. Germany

D. America.

3. In the last paragraph, the underlined word "which"refers to _____.

A. family life

B. situations

C. other values

D. trends

4. What message can we get from the text?

A. The GDP of Europe is higher than that of America.

B. Two possible reasons are given for working longer hours in the US.

C. People all over the world choose to work less when they are richer.

D. Americans are happier than Europeans.

5. Which of the following would be the best title for the text?

A. Americans and Europeans

B. Staying at Home

C. Work and Productivity

D. Work and Happiness

单项选择题