一辆汽车以12m/s的速度行驶，遇到紧急情况，司机刹车，使汽车做匀减速直线运动，若制动后汽车的加速度大小为6m/s2，则下列判断正确的是（ ）
A．开始刹车经3秒，汽车的速度大小为6 m/s
B．开始刹车经3秒，汽车的位移大小为9m
C．开始刹车经3秒，汽车的速度大小为0
D．开始刹车经3秒，汽车的位移大小为12m
A、汽车制动到停止的时间t0=
=2s．所以经过3s后，汽车已停止，所以速度为0．故A错误，C正确．△v △t
B、经过2s汽车停止，所以3s内的位移等于2s内的位移，则x=v0t0+
at02=12×2-1 2
×（-6）×4m=12m．故B错误，D正确．1 2
故选CD．
Complete the following passage by using the words in the box. Each word can be used only once. Note that there is one word more than you need.
 rce ___4___ their knowledge, not their muscles. Knowledge workers get paid for their education and their ability to learn. Knowledge workers ___5___ in mind work. They deal with symbols: words, figures, and data.What does all this mean for you? As a future knowledge worker, you can expect to be communicating information. Currently, many jobs ___6___ some form of mind work, and that number will increase sharply in the future. In the new world of work, you can look forward to being in __7___ training to get new skills that will help you keep up with improved technologies. You can also expect to be taking greater control of your career. Gone are the nine-to-five jobs, lifetime security, easy promotions, and even the ___8___ workplace, which you are __9___ with now. Don’t expect the companies will provide you with a clear career path. And don’t wait for someone to “encourage” you. You have to encourage yourself. |