问题 问答题

信阳市某交通路口绿灯即将结束时会持续闪烁3s,而后才会变成黄灯,再经过3s黄灯提示后转为红灯.2013年1月1日实施新的交通规定:黄灯亮时车头已经越过停车线的车辆可以继续前行,车头未越过停车线的若继续前行则视为闯黄灯,属于交通违章行为.(刹车过程可视为匀减速直线运动)

(1)若某车在黄灯开始闪烁时刹车,要使车在黄灯提示的时间内停下来且刹车距离不得大于18m,则该车刹车前的行驶速度不能超过多少?

(2)若某车正以v0=15m/s的速度驶向路口,此时车距停车线的距离L=48.75m,当驾驶员看到绿灯开始闪烁时,经短暂考虑后开始刹车,该车在红灯刚亮时恰好停在停车线以内,求该车驾驶员的考虑时间.

答案

(1)设在满足题设条件的情况下该车的最大行驶速度为v,根据平均速度公式,有:x1=

v
2
t1 

代入数据解得:v=12m/s 

(2)设该车驾驶员的反应时间为t,在反应时间内车行驶的距离为L0,则:

L0=v0t 

从绿灯闪烁到红灯亮起的过程中,汽车做减速运动的时间为:t2=6-t 

汽车在刹车过程中通过的位移为:x2=

v0
2
t2 

绿灯开始闪烁时,该车距停车线距离为:L=L0+x2 

联立并代入数据解得:t=0.5s 

答:(1)该车刹车前的行驶速度不能超过12m/s. 

(2)该车驾驶员的考虑时间为0.5s.

阅读理解

阅读理解。

   Have you ever dreamed about a trip to space? Seeds (种子)of some vegetables and crops are a lot

luckier than you, as they have travelled to space and back.

    China is among the three countries in the world that carry out agricultural (农业的)research in space.

The others are Russia and the U.S. Since 1987, China has sent about 1000 kinds of seeds into space.

These include green peppers, tomatoes, cucumbers, carrots and rice. Even on China's first manned spacecraft Shenzhou V there were some seeds from Taiwan.

    "Conditions(条件) in outer space such as radiation(辐射)and low gravity can cause big changes in the

seeds," said Chinese researcher Jin Xing.

    But not all seeds are better after going to space. After returning to the earth, the best seeds are chosen. These seeds produce much more and also grow faster than normal seeds. As the safety of genetically

modified(转基因)food is still being talked about, some people are not sure about the safety of these

"space vegetables" because some of their genes(基因)are changed during the trip to space.

   But Jin holds the more hopeful view. He said, "Unlike genetically modified crops, no new genes are

introduced to the space vegetables, so they are surely safe."

    Space vegetables are better than normal vegetables. For example, space tomatoes stay fresh for twenty

days, which is one week longer than normal tomatoes. Another popular space vegetable is the peppers.

Space peppers have 20 per cent more vitamin C than normal peppers.

    Researchers are considering holding a tomato festival. By the time people will be able to enjoy about 30 different kinds of space tomatoes on their dining tables.

So far, the Chinese Academy of Sciences has set up three space vegetable base fields, one of which is in

Shenzhen while the others are in Beijing and Jiangsu Province.

1. What will happen when seeds of some vegetables are sent to space and carried back?

A. Some of their genes are changed.      

B. They are all better than before going to space

C. No changes are caused in the seeds.    

D. They all produce much more and grow faster.

2. Why does Jin Xing have no doubt about the safety of the space vegetables?

A. Because the seeds sent to space stay the same.

B. Because the seeds sent to space are the best.

C. Because no new genes are brought into them.

D. Because space vegetables are luckier.

3. Which of the following statements is TRUE?

A. Genetically modified food is completely safe. 

B. The space vegetables don't belong to genetically modified food in some ways. 

C. About 1000 kinds of seeds were sent into space on Shenzhou V.  

D. Seeds back from space must be planted in special rooms controlled by computers.

4. What is the underlined word "cucumber" in the second paragraph?

A. A product from Russia.            

B. A kind of gene. . 

C. A product from Taiwan.          

D. A kind of agricultural product.

判断题