问题 选择题

炮弹发射后在空中飞行,若不计空气阻力,下列说法中正确的是(  )

①炮弹只受重力作用,方向沿竖直方向向下

②若炮弹的质量是m,则其重力大小为mg

③炮弹在飞行中还受到一个向前的推力,其方向不断发生变化

④炮弹在飞行过程中还受到大炮对它的推力作用.

A.①②

B.③④

C.①③

D.②④

答案

①炮弹只受重力作用,方向沿竖直方向向下,故①正确

②若炮弹的质量是m,则其重力大小为mg,故②正确

③炮弹在飞行中不受一个向前的推力,力不能离开物体而存在.故③错误

④炮弹在飞行过程中不受到大炮对它的推力作用,是在发射瞬间受到大炮对它的推力作用,给它很大的初速度,故④错误

故选A.

问答题
单项选择题

In bringing up children, every parent watches eagerly the child’s acquisition of each new skill—the first spoken words, the first independent steps, or the beginning of reading and writing. It is often tempting to hurry the child beyond his natural learning rate, but this can setup dangerous feelings of failure and states of worry in the child. This might happen at any stage. A baby might be forced to use a toilet too early; a young child might be encouraged to learn to read before he knows the meaning of the words he reads. On the other hand, though, if a child is left alone too much, or without any learning opportunities, he loses his natural enthusiasm for life and his desire to find out new things for himself.

Parents vary greatly in their degree of strictness towards their children. Some may be especially strict in money matters. Others are strict over times of coming home at night or punctuality tot meals. In general, the controls imposed represent the needs of the parents and the values of the community as much as the child’s own happiness.

As regards the development of moral standards in the growing child, consistency is very important in parental teaching. To forbid a thing one day and excuse it the next is no foundation for morality. Also, parents should realize that "example is better than precept". If they are not sincere and do not practise what they preach, their children may grow confused, and emotionally insecure when they grow old enough to think for themselves, and realize they have been to some extent fooled.

A sudden awareness of a marked difference between their parents’ principles and their morals can be a dangerous disappointment.

In moral matters, parents should ().

A.observe the rules themselves

B.be aware of the marked difference between adults and children

C.forbid things which have no foundation in morality

D.consistently ensure the security of their children