问题 选择题

如图所示,粗糙水平面上放置有。一个滑块,质量为M,其内部带有一光滑的半圆形凹槽;一质量为m的小球在凹槽内部往复运动,滑块始终静止不动;在小球由静止开始从凹槽右端最高点滑向最低点的过程中,下列说法正确的是(   )

A.地面对滑块的摩擦力方向向左

B.小球始终处于超重状态

C.地面对滑块的支持力大小等于(M+m)g

D.小球重力的功率逐渐增大

答案

答案:A

小球沿光滑曲面下滑的过程中,受到重力和支持力,只有重力做功,机械能守恒,势能减小,故动能增加;对小球和滑块整体受力分析,根据牛顿第二定律列式分析地面对滑块的摩擦力和支持力的情况;根据瞬时功率表达式判断瞬时功率情况.

解:A、C、小球沿光滑曲面下滑的过程中,速度不断增加,对整体受力分析,受到重力、支持力和向左的静摩擦力,如图

根据牛顿第二定律,有

f=max

(M+m)g-N=may

解得

N=(M+m)g-may<(M+m)g

故A正确,C错误;

B、小球加速下降,处于失重状态,故B错误;

D、小球重力的功率等于重力和重力方向的分速度的乘积,由于最低点速度水平,竖直方向的分速度为零,故小球加速下滑到最低点的过程中,竖直方向的分速度先增大,和减小,故重力的瞬时功率先变大后变小,故D错误;

故选A.

单项选择题 A3/A4型题
阅读理解

The United Nations says forty million people or so around the world went hungry in 2008,mainly because of higher food prices. Early estimates from the UN Food and Agriculture Organization (FAO) show that 963 million people did not get enough to eat.

World food prices have dropped since early 2008.Prices of major crops have decreased by more than half from their height earlier last year. But they remain high compared to earlier years.

But FAO official Hafez Ghana says lower prices have failed to end the food crisis(危机)in many poor countries."For millions in developing countries," he says, "getting enough food every day to live an active and healthy life is a distant dream.”

The FAO says food shortage is a threat to people's health.Today, two-thirds of the world's undernourished people live in just a few countries. These are India, the Democratic Republic of Congo, Bangladesh, Indonesia, Pakistan, Ethiopia and so on.

A report on food insecurity warns that the current economic crisis could send even more people into hunger and poverty.

In sub-Saharan Africa, the percentage of the people who continually go hungry fell from 34% in 1997 to 30% in 2008.But the FAO says Ghana is the only country that has reached two sets of hunger reduction targets.These were set by the 1996 World Food Summit and the Millennium Development Goals.The main reason is the growth in agricultural production in Ghana.

The FAO says some countries in Southeast Asia like Thailand and Vietnam have made progress in hunger reduction goals.But South Asia and Central Asia haven't, and North Korea is still in hot water.

31. What FAO official Hafez Ghana says implies _.

A. it's easy but takes long to provide people with enough food

B. enough food can make people more active and healthier

C. there is difficulty solving the food shortage in a short time

D. people in developing countries will never get enough food

32. Ghana has reached the targets of hunger reduction mainly because of _.

A. the still high food prices                      

B. the donation of developed countries

C. the two targets of hunger reduction      

D. the growth in agricultural production

33. The underlined word "undernourished" in Para. 4 probably means _.

A. hungry and unhappy                       B. unhealthy for lack of food

C. not fat because of poverty               D. undeveloped and poor

34. Which country has not made progress in hunger reduction?

A. North Korea    B. Thailand      C. Vietnam     D. Ghana

35. What is the best title of this passage?

A. The food production of the world

B.    The hunger reduction target of the FAO

C.    The food shortage around the world

D.    The solution to the global food shortage