以速度为v0=10m/s匀速行驶的汽车在第2s末关闭发动机，以后作匀减速直线运动，第3s内的平均速度是9m/s，试求：
（1）汽车作减速直线运动的加速度a；
（2）汽车在10s内的位移s．
（1）设汽车在第3s末的速度为v1，由题意有：
=. v3 v0+v1 2
解得：v1=8m/s
因此：a=
=-2m/s2 v1-v0 t
（2）汽车在关闭发动机前2s内的位移为：x1=v0t1=20m
汽车在关闭发动机后速度减为零所需时间：t2=
=5s△v a
因为10s＞5s，所以汽车关闭发动机5s后就停止运动，
故汽车在关闭发动机后的位移为：x2=
t2=25mv0 2
因此，汽车在前10s内的位移为：x=x1+x2=45m
答：（1）汽车作减速直线运动的加速度为=-2m/s2；
（2）汽车在10s内的位移为45m．
| 完型填空。 | |||
| Amy is 13. She lives __1__ her parents in Suzhou. She is in Grade 7. From Monday __2__ Friday, she goes to school. And she usually __3__ school clothes. There are lots of nice people in her class. Millie her best friend, always __4__ games with her at lunchtime. They both enjoy __5__ the songs of Jay Zhou. She is very busy now. Tomorrow is New Year's Day. They are going to __6__ it in their classroom, __7__ Amy and Millie are going to the supermarket. They want to buy some __8__: sweets, chocolates, lemons and pears for their classmates. Amy often ate chocolates before, but now she __9__ eat them any more because she wants to be healthier. Millie loves pears. She is going to buy five __10_ of them. | |||
| ( )1. A. and ( )2. A. on ( )3. A. in ( )4. A. plays ( )5. A. listening ( )6. A. celebrate ( )7. A. because ( )8. A. food ( )9. A. don't ( )10. A. pieces | B. with B. at B. wear B. play B. hearing B. wait for B. so B. clothes B. doesn't B. bottles | C. without C. to C. put on C. playing C. listening to C. talk about C. but C. drink C. isn't C. pairs | D. in D. and D. wears D. played D. listen to D. sing D. and D. books D. didn't D. kilos |
