问题 问答题

如图所示,光滑水平面上有辆静止的绝缘小车,车上固定两块竖直带电金属板A、B,A、B间距为d,其间匀强电场的强度为E,方向水平向左,车和A、B的总质量为M,今有一个质量为m、带电量为+q的微粒C,以水平向右的速度v0从金属板B的中心小孔射入电场中,欲使带电微粒C不打到A板上,v0必须满足的条件?

答案

带电量为+q的微粒C进入电场中,做减速运动,

欲使带电微粒C不打到A板上,临界状态是微粒C到达A板时,微粒C和A板具有相同的速度.

设微粒C刚好到达A板时,微粒C和A板具有相同的速度是v,

车和A、B、C所组成的系统在水平方向上合力为零,动量守恒,规定向右为正方向,列出等式:

mv0=(M+m)v

v=

mv0
m+M
…①

微粒从金属板B的中心小孔射入电场到A板,电势增大U=Ed,所以电势能增大:Uq=Edq.

由于系统的动能减小转化为微粒C的电势能,根据能量守恒列出等式:

1
2
m
v20
-
1
2
(M+m)v2=Edq…②,

由①②得:v0=

2(M+m)Eqd
Mm

所以v0满足:v0

2(M+m)Eqd
Mm

答:欲使带电微粒C不打到A板上,v0必须满足的条件是v0

2(M+m)Eqd
Mm

阅读理解

In bringing up children, every parent watches eagerly the child's acquisition (学会) 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 set up 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 severe over time of coming home at night or punctuality(准时)for meals. In general, the controls 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(前后一致) is very important in parental teaching. To forbid a thing one day and excuse it the next is no foundation for morality (道德). Also, parents should realize that "example is better than precept". If they are not sincere and do not practice what they preach (说教), their children may grow confused 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.

小题1:Eagerly watching the child’s acquisition of new skills, _________________.

A.should be avoided

B.is universal among parents

C.sets up dangerous states of worry in the child

D.will make the child lose interest in learning new things小题2:In the process of children’s learning new skills, parents _________________.

A.should encourage them to read before they know the meaning of the words they read

B.should expect a lot of the children

C.should achieve a balance between pushing them too hard and leaving them on their own

D.should create as many learning opportunities as possible小题3:The second paragraph mainly tells us that _________________.

A.parents should be strict with their children

B.parental controls reflect only the values of the community

C.parental restrictions vary, and are not always for the benefit of the children alone

D.it’s parents’ and society’s duty to control the children小题4:The word“precept” in Paragraph 3 probably means “_________________”.

A.opinion

B.punishment

C.behavior

D.instruction小题5:In terms of moral matters, parents should _____________________.

A.follow the rules themselves

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

C.forbid things which have no foundation in morality

D.always ensure the security of their children

单项选择题