题目

如图是一个由两个定滑轮和一个动滑轮组成的滑轮组，站在地上的人用力向下拉绳，提起重为600N的物体。（1）在图中画出滑轮组的绕绳方案（要求三个滑轮都要使用）。（____）（2）若不计摩擦和滑轮的重，所用的拉力为多大__？（3）若不计摩擦，实际所用拉力为220N，求动滑轮的总重__。 答案：【答案】 200N 60N 【解析】滑轮组的省力特点：滑轮组由几段绳子承担物重，拉起绳子的力就是物重的几分之一．（1）此题可以首先确定绳端的用力方向，然后依次绕线．（2）首先判断此滑轮组由几段绳子承担物重，若不计绳重和摩擦，则F=，后根据F=来计算拉力的大小．（3）若不计摩擦，用到滑轮组When the TV viewer turns on his set, what sort of programs does he have to choose from? You might think there would be more programs devoted to entertainment than to anything else, but that’s not the case. In most countries, fewer than 20% of broadcasting hours are devoted to entertainment. U.S. figures are high----34.8% , and the unloving Canadians are even higher with 44%. Except Canada and Italy, all countries give more broadcasting time to education than to either information (news , documentaries and so on) or entertainment programs. Of course, few educational broadcasts take place during peak viewing times. In Japan though , more than 60%of broadcasting time is taken up with education of one kind of anther----just another example of the businesslike Japanese philosophy. In the U.K., the figure is 56.4% . the Italians have fewer educational programs than anyone else. They don’t go in for entertainment either. Only about ten percent of viewing time is devoted to dramas and serials, quiz shows, music, sports etc. You will find more news information programs on Italian TV than anything else. That’s understandable in a country experiencing social and political changes. Italians rely on TV to tell them what’s going on---and events are happening almost too fast to follow. The percentage of time the U.S. devoted to news and documentary programs is much smaller. After education, most TV time is given to entertainment. Many of these programs are shown around the world.1.Based on this passage, the greatest percentage of TV broadcasting hours to educational is in __.A.JapanB.ItalyC.CanadaD.the United States2.More news information programs are broadcast on Italian TV than anywhere else because the Italians ___.A.are interested in what is happening in the world.B.Like to follow the changes that are going on in their social lifeC.Prefer to learn news information on TV rather than in newspapersD.Expect TV to tell them the latest news about what is going on in their country3.So far as the broadcasting hours devoted to entertainment , which of the following is true?A.The Japanese figure is the highest in the world.B.The U.S. figure is smaller than the U. K. figureC.The U.K. figure is second to the Japanese figureD.The Canadian figure is higher than that of any other country.4.In the United States, ____.A.TV programs are shown for world audience to watchB.Most of TV broadcasting hours are give to entertainmentC.Educational programs are shown during peak viewing timesD.TV broadcasting hours devoted to education are more than those devoted to entertainment