历史上曾把在相等位移内速度变化相等的单向直线运动称为“匀变速运动”．现称“另类匀变速直线运动”，另类匀变速直线运动的加速度定义为A=
，其中v0和vt分别表示某段位移s内的初速度和末速度；而现在物理学中加速度的定义式为a=vt-v0 s
．（ ）vt-v0 t
A．若A不变，则a也不变
B．若A＞0且保持不变，则a逐渐变大
C．若A＞0且保持不变，则a逐渐变小
D．若A不变，则物体在中间位置处的速度为v0+vt 2
A、B、C、若A不变，有两种情况一是：A＞0，在这种情况下，相等位移内速度增加量相等，所以平均速度来越大，所以相等位移内用的时间越来越少，由a=
可知，a越来越大．故A、C错误，B正确．vt-v0 t
D、因为相等位移内速度变化相等，所以中间位置处位移为
，速度变化量为s 2
，所以此位置的速度为v0+vt-v0 2
=vt-v0 2
．故D正确．v0+vt 2
故选BD．
| Complete the following passage by using the words in the box. Each word can only be used once. Note that there is one word more than you need. | |
Money is the root of all evil- and new study claims there may be some truth behind the saying. Scientists at the University of California, Berkeley, US, 1 on February 27 that rich people are more likely to do 2 things, such as lie or cheat, than poorer people. The scientists did a series of eight experiments. They 3 their findings online in the Proceedings of the National Academy of Sciences (PNAS). They carried out the first two experiments from the sidewalk near Berkeley. They noted that drivers of newer and more expensive cars were more likely to cut off other cars and pedestrians at crosswalks. Nearly 45 percent of people driving expensive cars 4 a pedestrian compared with only 30 percent of people driving more 5 cars. In another experiment, a group of college students was asked if they would do unethical(不道 德的) things in various everyday situations and 6 included taking printer paper from work and not telling a salesperson when he or she gave back more 7 . Students from higher-class families were more likely to act dishonestly. According to the scientists, rich people often think money can get them out of trouble. This makes them less afraid to take risks. It also means they care less about other people's feelings. Finally, it 8 makes them greedier . "Higher wealth status seems to make you want even more, and that increased want leads you to bend the rules or break the rules to serve your self-interest," said Paul Piff, lead scientist of the study. Piff pointed out that the findings don't mean that all rich people are untrustworthy or all poor peoplehonest. He said the experiments were to show how people living in different social situations express their 9 and values in different ways. |