问题 选择题

如图所示,在竖直平面内有一半径为R的圆弧轨道,半径OA水平、OB竖直,一个质量为m的小球自A的正上方P点由静止开始自由下落,小球沿轨道到达最高点B时恰好对轨道没有压力.已知AP=2R,重力加速度为g,则小球从P到B的运动过程中(  )

A.重力做功2mgR

B.小球机械能守恒

C.合外力做功mgR

D.克服摩擦力做功

1
2mgR

答案

A、重力做功WG=mg(2R-R)=mgR,故A错误;

B、小球沿轨道到达最高点B时恰好对轨道没有压力,则有

mg=m

v2
R

解得:v=

gR

根据动能定理得:

W=

1
2
mv2=
1
2
mgR

合外力做的功不等于重力做的功,所以机械能不守恒,故BC错误;

D、WG+Wf=

1
2
mv2=
1
2
mgR

所以Wf=-

1
2
mgR

所以克服摩擦力做功为

1
2
mgR,故D正确.

故选D

单项选择题

Some politicians are scurrying about with much zest and anticipation. It’s time, their polls inform them, to find the quick fix for what they have determined is a society plagued by the irregular heartbeat of deficient values.

But there are contradictions that intrude on this denunciatory atmosphere. If there are moral omissions in the society, they cannot be sealed by instant, slenderly based attacks on entertainment. The plain fain fact is we are rearranging our priorities in the wrong way.

We are today misplacing our energies and our funding by directing all sorts of incentives to high schools and colleges. Too late. The moral scaffolding has been built by then, for better or worse. How then to begin this revision of life conduct We must introduce in pre-school, and keep alive through grade five, a new school course.

The course could be titled, "What is right, and what is plainly wrong. " For 30 minutes each day, the teacher would illuminate for these very young children what William Faulkner labeled "the old verities", the words that construct and implement the daily moral grind in every durable society must engage if it is to be judged a "just" society.

These are words like duty, honor, service, integrity, pity, pride, compassion and sacrifice, plus the clear admonition that violence is wrong. To the teaching of the meaning of those words must be added that cleansing rule of treating other people as you would want them to treat you. And most of all to make sure that these kids understand with growing clarity that home, school and church are the sanctuaries for their later life.

There is a grand simplicity to this kind of school course. It enters a child’s mind early, burrowing deep into those recesses of the human brain that even today advanced medical science has not been able to penetrate.

If you ask enough people, you will find that most of us remember our first-or second-grade teacher. I remember Miss Corbett and Miss Walker, who read to us before we really understood, but the words had weight and allure. We listened and, without really knowing it, we learned and saved what we learned. Perhaps it was because what we heard in those early school years was the first entry into our learning vessel.

Absent this kind of early instruction, absent the building of this moral shield, no congressional law, no presidential executive order, no fiery rhetoric will salvage a child’s conduct nor locate a missing moral core.

"The old verities" labeled by William Faulkner must refer to ().

A. the past ways for ruling a society

B. the society’s established virtues

C. the out-of-date moral conducts

D. the previous legal systems

单项选择题