将质量为0.5kg的小球以14m/s的初速度竖直向上抛出，运动中小球受到的空气阻力大小恒为2.1N．
求：（1）小球上升的加速度大小
（2）小球能上升的最大高度．
（1）小球受到重力和向下的阻力，根据牛顿第二定律，有：mg+f=ma
解得：a=g+
=9.8+F m
=14m/s22.1 0.5
（2）根据速度位移关系公式，有：
h=
=v2 2a
=7m142 2×14
答：（1）小球上升的加速度大小为14 m/s2；
（2）小球能上升的最大高度为7m．
| 阅读理解。 |
| People have enjoyed the benefits (好处) of cycling in Amsterdam for years. It is a good city for cycling because it is flat (平坦) and so is convenient for bikes. There are also plenty of places for bicycle parking and most streets in the city centre have a bicycle path. Today some people call Amsterdam the "City of Bicycles: because it's convenient for people to cycle there." In the 1960s, a group of cycling fans had an idea. They believed that it would be better for everybody if cars weren't allowed in the city centre and only bicycles were. They were hopeful that this would help to save energy, reduce pollution and provide free public transport. The group painted hundreds of bicycles white and placed them in lots of neighborhoods around Amsterdam for people to use. Anyone was allowed to take them and use them for short journeys. When someone finished a journey they would leave the bike there for someone else to use. The problem was that it didn't work-thieves took all the bicycles within weeks! However, more than thirty years later, the "white bike" is back in town this time with a computer chip to record its every move! To take a bicycle, you have to put a special card inside. The new"white bike" is not actually white but is an unusual design with bright colors. The bikes are parked at special parking place that has enough room. There is already less traffic in central Amsterdam, because both locals and tourists have used the white bikes. Indeed, thanks to the good ideas of lots of people, like the cycling fans in the 1960, many people around the world have been enjoying city centre streets without cars for many years. |
| 1. Why Amsterdam is called the "City of Bicycles"? |
[ ] |
| A. Because it's convenient for people to cycle there. B. Because there are so many bicycles there. C. Because most streets in the city center have a bicycle path. D. Because there are lots of places for bicycle parking. |
| 2. The first "white bikes" plan failed because _____. |
[ ] |
| A. all the bicycles were white B. no enough bicycles were provided C. thieves took the bicycles D. people couldn't rightly park the bicycles |
| 3. How does the "white bike" plan work thirty years later? |
[ ] |
| A. All the bikes are parked at special parking places. B. All the bikes should be kept properly by special people C. Enough room is left for keeping bicycle. D. By using both computer and special parking places |
| 4. Which of the following is the best title? |
[ ] |
| A. Thanks to the good ideas! B. Return of the white bikes! C. White bikes or not! D. No cars in Amsterdam! |