题目

物体从静止开始做匀加速直线运动，第3s内通过的位移是3m，则（ ）A. 前3s的位移是6m B. 3s末的速度是3.6m/sC. 3s内平均速度是2m/s D. 第3s内平均速度是3m/s 答案：【答案】BD【解析】从静止开始做匀加速直线的物体在相等时间内走过的位移比为1:3：5，又因为第3s内通过的位移是3m，所以第1s内的位移是，第2s内的位移是，所以3s内的位移是5.4m，A错误，匀加速直线运动的物体在相等时间内走过的位移差是一个定值即,从题中可知在每个1s内走过的位移差为，所以，所   Do you know the cold? Everybody knows what happens when you catch a cold. First you sniffle(抽鼻涕) or get a more throat. Then you sneeze. After coughing and blowing your way through the next few days it’s over---until next time Do you know what causes a cold, how it is spread or how to help prevent it? And which of the many medicines is best for your cold? There is no known cure for the common cold. Once you catch a cold, your body usually produces antibodies(抗体) to the virus which caused the cold. These antibodies help the body develop resistance（抵抗力) to the virus if you catch it again. Unfortunately, over 200 different viruses cause the common cold. These viruses may change over time, so your resistance to them reduces. The large number of viruses and the fact that they change help explain why there is no cure for the common cold and why you will suffer colds in the future. The colds increase in winter(usually between Septenber and March) but Not because of the cold! Cold weather has not been shown to increase the number of colds. People spend more time indoors in winter. They are therefore more likely to come into physical contact with each other and spread the disease. All these ideas remain to be proven. No one has been able to discover what really causes these colds Common colds are spread by sneezes or by direct contact with a cold sufferer, usually when two people’ hands touch or an object is passed from one person to another. Sneezing spreads colds when someone sneezes near other people, or sneezes on objects which others later touch. A cold virus can survive up to three hours on hard surfaces. The simplest and most effective way to lower the risk of catching a cold is to wash your hands frequently. Another important step in cold prevention is for cold sufferers to get rid of used tissues immediately after use. 1.From the passage, we can know____________. A. a cold virus can survive up to four hours       B. most of people know how to cure a cold C. there are more colds in November than in May   D. all of us catch more than one cold a year 2.According to the passage, people tend to catch a cold___________. A. because they don’t take exercise in winter    B.if they stay outdoors in freezinf cold winter C. if they stay indoors more often in winter   D.because the viruses of colds usually exist in winter 3.We can know colds sre spread through____________. A. air and touching     B. coughs and objects   C.two people    D. a person and his hands