课外阅读。

祖国山河多美丽

　　轰隆隆，轰隆隆。火车像骏马，在原野上跑得欢。车窗像照相机，拍下了一张张活动的照片。

　　一张照片是田野。田野好大好大，大得望不到边。一条条大道，把绿色的田野分成一块块的。

　　一张照片是高山。山好高好高，高得望不到顶。一座座高山，手牵着手，肩挨着肩。

　　一张照片是绿水，水面好宽好宽，宽得望不到岸。渔船星星点点，渔民撒网捕鱼。
　　轰隆隆，轰隆隆。火车像骏马，在原野上跑得欢。山山水水看不完，祖国山水多美丽。

一个物体做直线运动，它在前2s的平均速度是6m/s，后4s内的平均速度是3m/s，则该物体在6s内的平均速度是____ m/s。

①此时气体的温度为多少；

②在对缸内气体加热的过程中，气体膨胀对外做功，同时吸收 $Q=390J$ 的热量，则气体增加的内能 $△U$ 多大．

    Many years ago, I was a shy boy and lived a very peaceful life. I liked school but I felt bored.

    And then I started the fourth grade. My teacher, Mrs. Chen, was a young woman who was good at writing, art and music. Her class was very interesting. With her help, I started Fun Club with some boys in my class. I also wrote a book called Through the Forest. And lots of my classmates liked it.

    However, I was surprised when fifty years later I got a letter about a reunion (聚会). A few days later, we had a reunion at Mrs. Chen's home. Reconnecting with Mrs. Chen was a journey back in time. Fifty years have passed, but my teacher still remembers every student. She even doesn't forget the class bully (欺负) — a boy used to hit me with snowballs on my way to school. Lots of teachers thought bullying was nothing serious at that time, but Mrs. Chen didn't agree. She put a stop to it at once when she found out about it.

    Now, in my mid-fifties, I appreciate the gifts Mrs. Chen has given me even more. It's never too late to say thanks to this fantastic teacher. I hope you are lucky enough to have one teacher like Mrs. Chen.