题目

指尖陀螺是目前流行的一种玩具，陀螺的中间是轴承，轴承内有滚珠，边上有三个用密度较大的金属制作的飞叶，当陀螺水平放置在指尖上，拨动飞叶后，飞叶可以绕竖直轴在指尖上长时间转动，飞叶旋转时，下列分析正确的是( )A.转速变慢时惯性变小B.轴承内滚珠可减小摩擦C.陀螺的重力和它对指尖的压力是一对平衡力D.飞叶转动后不能立即停下来是由于受到惯性作用 In the 13th century, the famous Italian traveler, Marco Polo, traveled a long way to China. During his stay in China, he discovered that the Chinese used paper money. In western countries, people did not use paper money until the 15th century.A Chinese man called Cai Lun invented paper almost 2000 years ago. He made it from wood.Now paper still comes from trees. We use a lot of paper every day. Every day, people throw away about 280 tons of paper in our city. It takes 17 trees to make one ton of paper. This means that we are cutting nearly 4800 trees every day. Since it takes more than 10 years for a tree to grow, we must start using less paper now. If we don’t, we will not have enough time to grow more trees to take the place of those we use for paper.So how can we save paper? We can use both sides of every piece of paper, especially when we are making notes. We can choose drinks in bottles instead of those in paper packets. We also use cotton handkerchiefs and not paper ones. When we go shopping, we can use fewer paper bags. If the shop assistant does give us a paper bag, we can save it and reuse it later.1.Who is Marco Polo?__________________________________________2.When did Cai Lun invent paper ?__________________________________________3.How many trees are people cutting every day?__________________________________________4.Will we have enough time to grow more trees if we don’t save paper?__________________________________________5.How can we save paper when we go shopping? __________________________________________