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问题 选择题

有一个质量为m的小木块，由碗边滑向碗底，碗内表面是半径为R的圆弧，由于摩擦力的作用，木块运动的速率不变，下列说法正确的是（   ）

A．它的加速度为零

B．它所受合力提供向心力

C．它所受的合外力大小一定、方向时刻改变

D．它的加速度恒定

答案

BC

题目分析：物块下滑过程速率保持不变，做匀速圆周运动，加速度不等于零，合外力不等于零．物块所受合外力提供向心力，合外力的大小不变，向心加速度大小不变，但方向指向圆心，不断变化，故选BC

点评：物块下滑过程速率保持不变，做匀速圆周运动，加速度不等于零，合外力不等于零．合外力提供向心力，大小不变，向心加速度大小不变，方向指向圆心，随时间变化．

填空题

对于只在表的首、尾两端进行插入操作的线性表，宜采用的存储结构为 [3] 。
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单项选择题

Few people would defend the Victorian attitude to children, but if you were a parent in those days, at least you knew where you stood: children were to be seen and not heard. Freud and company did away with all that and parents have been bewildered ever since. The child’s happiness is all-important, the psychologists say, but what about the parents’ happiness Parents suffer continually from fear and guilt while their children gaily romp about pulling the place apart. A good "old-fashioned" spanking is out of the question: no modern child-rearing manual would permit such barbarity. The trouble is you are not allowed even to shout. Who knows what deep psychological wounds you might inflict The poor child may never recover from the dreadful traumatic experience. So it is that parents bend over backwards to avoid giving their children complexes which a hundred years ago hadn’t even been heard of. Certainly a child needs love, and a lot of it. But the excessive permissiveness of modern parents is surely doing more harm than good.
Psychologists have succeeded in undermining parents’ confidence in their own authority. And it hasn’t taken children long to get wind of the fact. In addition to the great modern classics on childcare, there are countless articles in magazines and newspapers. With so much unsolicited advice flying about, mum and dad just don’t know what to do any more. In the end, they do nothing at all. So, from early childhood, the kids are in charge and parents’ lives are regulated according to the needs of heir offspring. When the little dears develop into teenagers, they take complete control. Lax authority over the years makes adolescent rebellion against parents all the more violent. If the young people are going to have a party, for instance, parents are asked to leave the house. Their presence merely spoils the fun. What else can the poor parents do but obey’
Children are hardy creatures (far hardier than the psychologists would have us believe) and most of them survive the harmful influence of extreme permissiveness which is the normal condition in the modern household. But a great many do not. The spread of juvenile delinquency in our own age is largely due to parental laxity. Mother, believing that little Johnny can look after himself, is not at home when he returns from school, so little Johnny roams the streets. The dividing-line between permissiveness and sheer negligence is very fine ’indeed.
The psychologists have much to answer for. They should keep their mouths shut and let parents get on with the job. And if children are knocked about a little bit in the process, it may not really matter too much. At least this will help them to develop vigorous views of their own and give them something positive to react against. Perhaps there’s some truth in the idea that children who have had a surfeit of happiness in their childhood appear like stodgy puddings and fail to make a success of life.

What is implied in the first sentence

A. There is no defense for Victorian harshness.
B. Parents are grateful to Freud for his advice.
C. Parents can be too strict with their children.
D. Child-care books prove sensible and practical.
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