题目

足球以10m/s的速率飞来，又被运动员以10m/s的速率踢回，脚踢的时间为0.1秒，则此过程中足球的加速度为（规定初速度方向为正方向）（ ） A．0       B．﹣200 m/s2      C．200 m/s2   D．﹣100 m/s2 答案：解：规定初速度方向为正方向，可知初速度为：v0=10m/s，末速度为：v=﹣10m/s，由加速度定义式可得： ． 故B正确． 故选：B．补全对话A: do you get up?B: I get up at six the morning and I run. Then I get and have breakfast 7: 00. What about ?A: Me? I get up at nine o'clock.B: Nine o'clock?A: , I go to school in the afternoon.B: Well, do you usually do in the evening?A: I usually do homework in the evening.B: Me, too. I do my homework at 7: 00 and I TV at 9: 00.A: Oh, I don't. I go to early (早).