题干

 如图所示为一种蓄水箱的人工放水装置,AOB是以O点为转轴的轻质杠杆,AO呈水平状态,A、O两点间的距离为40cm.B、O两点间的水平距离为10cm,竖直距离为7cm,K是一轻质、横截面积为100cm2的盖板(恰好堵住出水口),它通过细绳与杠杆的A端相连,在水箱右侧水平地面上,重为600N的人通过滑轮组拉动系在B点呈竖直状态的绳子,可以控制出水口上盖板,若水箱水深为50cm,当盖板恰好要被拉起时,水平地面对人的支持力为490N,人对绳子的拉力为F1,绳子对B点的拉力为F2,滑轮组的机械效率为η,盖板的厚度、绳重与滑轮间的摩擦均可忽略不计,人对绳的拉力与人所受重力在同一直线上(g取10N/kg)求:

上一题 下一题 0.0难度 选择题 更新时间:2011-12-01 08:24:31

答案(点此获取答案解析)

同类题2

完形填空

    In the middle of a beautiful day, I was feeling lost, as if the world was crashing on me. didn't know which way to go, except to head out to the countryside. I went to the spot where I used to take my kids-when I still had kids. Now they had  1and were leaving.

I was sitting alone in my car when I saw a man in his seventies, whose problem appeared bigger than mine. He stood near the edge of the road,2a kite spindle(风筝线锭) in his hands. Apparently, lost in thought, he stared skyward. I 3my neck to see how successful he had been in 4 the clouds. Following his string with my eyes, I almost lost5 of the other end. Finally, the kite rested on what seemed to be the highest branch of the 6tree in the park.  Obviously,  that was his grandson's kite!  The kite was a replica (复制品) of a beautiful bald eagle, and its wings spread 70pen. Grandpa had only one 8to his problem: cut the string.

"You wanted it to fly as high as it could. Bill. didn't you?" "Yes, Grandpa,9 wanted to keep it forever. " "There just comes a time when the only thing you can do is to cut the string and let it go. Perhaps by doing that, when it flies very high like real eagles do, it will come back to us. " Grandpa said.

    I 10 how the old man cut the string to let fly the kite. As the two walked away, looked to the 11. There I saw my answer, too. Today I have to cut the final strings that kept my two boys 12my reach. Though I want to keep them forever, I have taught them to fly like an eagle. Maybe by doing this, when it's time to soar like real eagles do. one day they would come back to me.