问题 问答题

如图5-4-4所示,有一连通器,左右两管的横截面积均为S,内盛密度为ρ的液体.开始时两管内的液面高度差为h,若打开底部中央的阀门K,液体开始流动,最终两液面相平.在这一过程中,液体的重力势能变化了多少?是增加了还是减少了?如果是减少了,减少的重力势能到哪里去了?

图5-4-4

答案

重力势能减少了h2ρgS,减少的重力势能转化为系统的内能.

由于A、B两管截面积相等,液体是不可压缩的,所以B管中液面下降的高度和A管中液面上升的高度相同,液面最终静止在初始状态A管液面上方h处.因为物体的重力势能与过程无关,只与初末状态的位置有关,所以可以将过程简化,视为将B管中h高的液柱移动到A管中,达到液体最终静止的状态,而其他的液体的位置没有变化,对应的重力势能也没有变化,全部液体重力势能的变化,就是B管上部h长的液柱重力势能的减少.不难看出,B管中重力势能变化的部分液柱其重心的高度减小了:Δh=h,它的重力mg=hSρg,所以全部液体重力势能减少了|ΔEp|=mgΔh=(ρhSg)( h)=h2ρgS,减少的重力势能全部转化为系统的内能.

选择题
阅读理解

What’s your dream vacation? Watching wildlife in Kenya? Boating down the Amazon? Sunbathing in Malaysia? New chances are opening up all the time to explore the world. So we visit travel agents, compare packages and prices, and pay our money.

We know what our vacation costs us. But do we know what it might cost someone else? It’s true that many poorer countries now depend on tourism for foreign income. Unfortunately, though, tourism often harms the local people more than it helps them.

It might cost their homes and lands. In Myanmar, 5,200 people were forced to leave their homes among the pagodas(佛塔)in Bagan so that tourists could visit the pagodas.

Tourism might also cost the local people their livelihood and dignity. Local workers often find only menial(卑微的)jobs in the tourist industry. And most of the profits do not help the local economy. Instead, profits return to the tour operators in wealthier countries. When the Maasai people in Tanzania were driven from their lands, some moved to city slums. Others now make a little money selling souvenirs or posing for photos.

Problems like these were observed more than 20 years ago. But now some non-government organizations, tour operators and local governments are working together to begin correcting them. Tourists, too, are putting on the pressure.

The result is responsible tourism, or“ethical tourism.”Ethical tourism has people at its heart. New international agreements and codes of conduct can help protect the people’s lands, homes, economies and cultures. The beginnings are small, though, and the problems are complex.

But take heart. The good news is that everyone, including us, can play a part to help the local people in the places we visit. Tour operators and companies can help by making sure that local people work in good conditions and earn reasonable wages.

They can make it a point to use only locally owned hotels, restaurants and guide services. They can share profits fairly to help the local economy. And they can involve the local people in planning and managing tourism.

What can tourists do? First, we can ask tour companies to provide information about the conditions of local citizens. We can then make our choices and tell them why. And while we’re abroad, we can:

★Buy local foods and products, not imported ones.

★Pay a fair price for goods and services and not bargain for the cheapest price.

★Avoid flaunting(炫耀) wealth.

★Ask before taking photographs of people. They are not just part of the landscape! Let’s enjoy our vacation and make sure others do, too.

小题1:.

.What is probably the best title for the article?

A.Tourism Causes Bad Effects

B.Tourism Calls for Good Behavior

C.Vacations Bring a Lot of Fun

D.Vacations Cost More Than You Think小题2:.

The underlined phrase“take heart”means“     ”.

A.pay attention

B.take care

C.cheer up

D.calm down小题3:.

According to the passage, the writer thinks     .

A.tourism is not a promising industry.

B.dream vacations should be spent abroad

C.the problems caused by tourism are easy to settle.

D.tourists should respect local customs and culture