问题
计算题
如下图所示，在光滑的水平面上钉两个钉子A和B，相距20cm。用一根长1m的细绳，一端系一个质量为0.5kg的小球，另一端固定在钉子A上，开始时球与钉子A、B在一直线上，然后使小球以2m/s的速率开始在水平面内做匀速圆周运动。若绳子能承受的最大拉力为4N，那么从开始到绳断所经历的时间是多少?
答案
3.768s
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如下图所示，在光滑的水平面上钉两个钉子A和B，相距20cm。用一根长1m的细绳，一端系一个质量为0.5kg的小球，另一端固定在钉子A上，开始时球与钉子A、B在一直线上，然后使小球以2m/s的速率开始在水平面内做匀速圆周运动。若绳子能承受的最大拉力为4N，那么从开始到绳断所经历的时间是多少?
3.768s
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| More than twenty tons of ivory (象牙), seized from illegal hunters, was burned in Kenya to keep the ivory off the international market and to discourage the illegal killing of elephants for their ivory. The past 12 months had seen a record number of large ivory seizures across the world, a wildlife protection organization said Thursday, saying it had been a horrible year for elephants. There were at least 13 large seizures in 2011, totalling at least 23 tons of ivory meaning that about 2,500 elephants were killed. This compares to just six large seizures in 2010, weighing a total of just under 10 tons, and confirms a sharp rise in the trade since 2007. This is the worst year ever for large ivory seizures in the past 23 years-2011 had truly been a horrible year for elephants. Most illegal shipments of African elephant ivory end up in either China, where it is made into powder and used in traditional medicine, or in Thailand, with Malaysia the most frequent transit country (中转 国). The increasing quantities of ivory being traded, most of which are from either Kenyan or Tanzanian ports, reflected a rising demand in Asia. They constantly change their transport ways to Asia to avoid detection, including switching from airplanes to seaships, and once the ivory products arrive, their documentation (证明文件) is rewritten to hide the fact that they came from Africa. As most large ivory seizures fail to result in any arrests, the criminals are winning and have illegally made large amounts of money. International trade in elephant ivory was banned in 1990, and ETIS (the Elephant Trade Information System) holds the details of reports on more than 17,000 ivory and other elephant product seizures across the world since then.
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