题目

如图所示，一块质量为M=2kg，长为L的均质板静止在很长的光滑水平桌面上，板的左端静止摆放质量为m=1kg的小物体（可视为质点），M和m之间的为动摩擦因数为μ=0.2．t=0s时刻在小物体m上施加一恒定水平拉力F=4N．t1=3s时撤去拉力，最后小物体m刚好能到长木板M的最右端，g取10m/s2．求： （1） 撤去拉力时，M和m的速度各是多少？ （2） 当 m到达长木板M右端时的速度是多少？ （3） 长木板M的长度L是多少？Holding a cell phone against your ear or storing it in your pocket may be dangerous to your health. This explains a warning that cell phone manufacturers include in the small print that is often ignored when a new phone is purchased. Apple, for example, doesn’t want iPhones to come closer to you than 1.5 centimeters; Research In Motion, BlackBerry’s manufacturer, recommends 2.5 centimeters. If health issues arise from cell phone use, the possible effects are huge. Voice calls - Americans chat on cell phones 2.26 trillion(万亿)minutes annually - earn $109 billion for the wireless carriers. Devra Davis, an expert who has worked for the University of Pittsburgh, has published a book about cell phone radiation, “Disconnect.” The book surveys scientific research and concludes the question is not settled. Brain cancer is a concern that Ms. Davis examines. Over all, there has not been an increase in its incidence since cell phones arrived. But the average masks an increase in brain cancer in the 20-to-29 age group and a drop for the older population. “Most cancers have multiple causes,” she says, but she points to laboratory research that suggests low-energy radiation could damage cells that could possibly lead to cancer. Children are more vulnerable（易受伤的）to radiation than adults, Ms. Davis and other scientists point out. Radiation that penetrates only five centimeters into the brain of an adult will reach much deeper into the brains of children because their skulls are thinner and their brains contain more absorptive fluid（易吸收的液体）. No studies have yet been completed on cell phone radiation and children, she says. Henry Lai, a research professor in the bioengineering department at the University of Washington, began laboratory radiation studies in 1980 and found that rats exposed to radiation had damaged DNA in their brains. Ms. Davis recommends using wired headsets or the phone’s speaker. Children should text rather than call, she said, and pregnant women should keep phones away from the abdomen（腹部）. 66. Why is the warning in the small print? A. They don’t want the users to pay attention to it. B. There is not enough space for the warning. C. They think people will not care about it. D. The warning is not important at all. 67. According to Ms Davis, brain cancer increase　　　 . A. among children　　　　　　　　　　　　 B. among old people C. in the twenties　　　　　　　　　　　　 D. among pregnant women 68. Why do children easily be affected by radiation? A. Because they haven’t grown up. B. Because they are too young to protect themselves. C. Because they use cell phones more often than adults. D. Because their skulls are thinner and their brains are easily hurt. 69. What can we conclude from the last paragraph? A. Pregnant women should keep cell phones away. B. People should use cell phones in the correct way. C. If you are a child, you’d better text than make phone calls. D. When you use a cell phone, use a wired headset or the phone’s speaker. 70. What does the passage mainly talk about? A. Be careful when using cell phones. B. Don’t hold your cell phone against your ear. C. Rats exposed to radiation have damaged DNA in their brains. D. Low-energy radiation could damage cells that could lead to cancer.