Some things can carry(携带) messages, for example, a sign（标志） at the bus stop helps you to know which bus to take. Signs on the door tell you where to go in or out. Have you ever noticed that there are a lot of signs around you and that you receive messages from them all the time?
   People can communicate on many other ways. An artist can use his drawing to tell about beautiful mountains, about the blue sea and many other things. Books are written to tell you about all the wonderful things in the world and also about people and their ideas.
   Books, magazines（杂志）, TVs, radios（广播） and films all help us to communicate（交流） with others. They all help us to know what is going on in the world and what other people are thinking about.

如图所示，甲、乙、丙三辆车行驶在平直公路上，车速分别为5m/s、8m/s、10m/s．当甲、乙、丙三车依次相距6m时，乙驾驶员发现甲车开始以1m/s²的加速度做匀减速运动，于是乙也立即做匀减速运动，丙车亦同样处理，直到三车都停下来时均未发生撞车事故．