题干

在小型建筑工地,常用简易的起重设备竖直吊运建筑材料,其工作原理相当于如图所示的滑轮组。某次将总重G为4000 N的砖块匀速吊运到高为10 m的楼上,用时40 s,卷扬机提供的拉力F为2500 N。求在此过程中:

(1)有用功;
(2)拉力F的功和功率;
(3)滑轮组的机械效率。

上一题 下一题 0.0难度 选择题 更新时间:2017-12-13 02:03:07

答案(点此获取答案解析)

解:题中使用的滑轮组,有2段绳子承担物重,绳子末端移动距离s=2h=2×10m=20m
(1)拉力做的有用功:W有用=Gh=4000N×10m=40000J;
(2)拉力做的总功:W=Fs=2500N×20m=50000J.
拉力做的总功的功率:

同类题1

阅读理解

阅读下列短文,从每题所给的四个选项 (A 、B 、C  和 D )中,选出最佳选项。

    Christmastime in the city brings forests of trees already cut and waiting to be sold. But some people like to drive to tree farms. Others wait for their trees to come to them. They order one from the pages of a catalog or on the Internet. Some say the easiest thing of all is to buy a man­made tree with Christmas lights already on it. No falling needles to have to clean up.

    The National Christmas Tree Association says 33,000,000 real trees were sold last year, compared to 9,000,000 man­made ones. Man­made trees generally cost more, but they can be reused. Most natural trees are cut up and recycled, but some people buy trees that can be planted.

    Most Christmas trees are now grown on farms instead of in forests. Twenty-one thousand tree farmers in the United States grow Christmas trees on more than 180,000 hectares. Oregon was the leading producer last year.

    Twenty-two percent of people who bought real trees last year chose them at a farm. Two percent of those people cut the trees themselves. The next most popular places were big stores like Walmart and Home Depot. Groups like the Boy Scouts also sell Christmas trees. But some people pay nothing for theirs. They steal it.

    Cornell University in Ithaca, New York, has many pretty evergreens. Some years ago, a university worker found a way to keep them there. A month before Christmas, workers treated them with “pink ugly mix”. It contains water and red food colour. The bright colour starts to disappear after about a month. It can take longer, however. Cornell decided not to use the mix this year, but the idea has spread.