一质点做单向直线运动。
（1）若前一半时间的平均速度为v1，后一半时间的平均速度为v2，则全程的平均速度为多大？
（2）若前一半位移的平均速度为v1'，后一半位移的平均速度为v2'，则全程的平均速度为多大？
解：（1）设全程的时间为2△t，则前一半时间内的位移△x1= v1△t，后一半时间内的位移△x2=v2△t，则全程位移△x=△x1+△x2=(v1+v2)△t，故全程平均速度
（2）设全程位移为2△x，则前一半位移的时间为
，后一半位移的时间为
，故全程平均速度为
| 完形填空。 | ||||
| Recently divorced in my 30s, I had no job and was on my way downtown on a rainy day to go to the employment offices. I had no umbrella, for my old one had 1 apart, and I couldn't afford another one. I sat down in the streetcar-there against the 2 was a beautiful silk umbrella with a silver handle inlaid with gold and bright enamel (珐琅). I had never seen anything so lovely. I 3 the handle and saw a name on it. The 4 procedure would have been to turn in the umbre la to the conductor, but I decided to take it and find the 5 myself. I got off the streetcar in a downpour and 6 opened the umbrella to protect myself. Then I searched a telephone book for the 7 on the umbrella and found it: I called and a lady answered. Yes, she said in 8 , that was her umbrella, which her parents, now dead, had given her as a birthday present, but it'd been 9 from her locker at school years before. She was so excited that I forgot I was looking for a job and went 10 to her house. She took the umbrella, her eyes filled with tears. I was only too happy to see her get back the lost umbrella, and get a job she offered me. | ||||
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