问题 选择题

如图所示,是同一滑轮组的两种不同的绕法,用它们提起相同

的重物,比较两种情况下所用的拉力F1、F2和机械效率η1、η2

(不计绳重及摩擦),下列说法中正确的是                              (   )

A.F1<F2、η1<η2

B.F1<F2、η1>η2

C.F1<F2、η1=η2

D.F1>F2、η1=η2

答案

答案:C

由图知,甲图n1=3,n2=2,

∵是同一滑轮组的两种不同绕法,用它们提起相同的重物,

∴G和G相同,

∵不计绳重及摩擦,甲图拉力F1=1/3(G+G),乙图拉力F2=1/2(G+G),

∴F1<F2;∵不计绳子重和摩擦,∴W=Gh,而W有用=Gh,

∵η==

∴η12.C对。

单项选择题

July 21,2007 was a typical English summer’s day—it rained for 24 hours ! As usual, I rushed home from work at midday to check on the house. Nothing was amiss. By the time I left work at 5 pm, however, the road into our village was flooded. Our house had never been flooded, but as I opened the front door, a wave of water greeted me. Thank God the kids weren’t with me, because the house was 5 feet deep in water. We lost everything downstairs. And the plaster of the walls, ceilings pulled down.

At first we tried to push on through. We didn’t want to move the children out of home, so we camped upstairs. We put a sheet of plastic across the floor to protect us from the damp. But after three months, we felt very sick, so we moved to a wooden house in a park. The house was small, but at first we were all just delighted to be in a new place. Unfortunately, things took longer than expected and we were there for 10 months. The life there was inconvenient. What surprised me most was how much I missed being part of a community. We had lived in a friendly village with good neighbors, and I’d never thought how much I’d miss that.

Although our situation was very hard, it’s difficult to feel too sorry for yourself when you look at what’s happening elsewhere. I watched a news report about floods in Northern India and thought, "We didn’t have a straw but that was swept away, and our house is still standing. We’re lucky. "

We moved back home in August. With December coming, there’s still reconstruction work to be done, so it’s difficult to prepare for Christmas. But I can’t wait—I’m going to throw a party for our friends in the village to say thanks for their support. This year, I won’t need any gifts—living away from home for months has made me realize how little we actually need or miss all our possessions. Although we are replacing things, there’s really no rush—we have our home back, and that’s the main thing.

What does the underlined word "amiss" in the first paragraph mean()

A.Wrong.

B.Missing.

C.Right.

D.Foun

填空题