问题 选择题

下图为两分子系统的势能Ep与两分子间距离r的关系曲线。下列说法正确的是(  )

A.当r大于r1时,分子间的作用力表现为引力

B.当r小于r1时,分子间的作用力表现为斥力

C.当r等于r2时,分子间的作用力为零

D.在r由r1变到r2的过程中,分子间的作用力做负功

答案

BC

题目分析:由图可知r2=r0,因此当r大于r1而小于r2时分子力为斥力,大于r2时分子力为引力,故A错误;由于r1<r2=r0,故当r小于r1时,分子间的作用力表现为斥力,故B正确.由于r2=r0,因此当r等于r2时,分子间的作用力为零,故C正确;当r由r1变到r2的过程中,分子力为斥力,因此分子间作用力做正功,故D错误.

故选BC.

点评:正确理解分子力、分子势能与分子之间距离的变化关系,注意分子力与分子势能变化的一个临界点为r=r0,注意将分子力与分子之间距离和分子势能与分子之间距离的图象比较进行学习.

单项选择题
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     Recently some American scientists have given a useful piece of advice to people in industrialized

nations.They say people should eat more of the same kind of food eaten by humans living more than

10,000 years ago.__1__

     The scientists say that the human life has changed greatly.Our bodies have not been able to deal

with these changes in lifestyle and this had led to new kinds of sicknesses.__2__ So they are called

"diseases of civilization".Many cancers and diseases of the blood system are examples of such

diseases.

     Scientists noted that people in both the Old Stone Age and the New Stone Age enjoyed very little

alcohol or tobacco,probably none.__3__ However,a change in food is one of the main differences

between life in ancient times and that of today.

     Stone Age people hunted wild animals for their meat,which had much less fat than domestic ones.

They ate a lot of fresh wild vegetables and fruits.They did not have milk or any other dairy products,

and they made very little use of grains.__4__ We eat six times more salt than our ancestors.We eat

more sugar.We eat twice as much fat but only one third as much protein and much less vitamin C.

__5__ But scientists say that we would be much healthier if we eat much the same way as our ancestors

did,cutting the amount of fatty,salty and sweet food.

A.These new sicknesses were not known in ancient times.

B.People today probably do not want to live the way people thousands of years ago did.

C.Ancient people also lived in large groups.

D.But today,we eat a lot of these food.

E.Ancient people also got a great deal of physical exercise.

F.In that case,they would live much healthier.

G.People today probably live the same life as people thousands of years ago.