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问题 计算题

静电场方向平行于x轴，其电势φ随x的分布可简化为如图所示的折线，图中φ0和d为已知量，一个带负电的粒子在电场中以x=0为中心、沿x轴方向做周期性运动。已知该粒子质量为m、电荷量为-q，其动能与电 势能之和为-A(0＜A＜qφ0)。忽略重力，求：

(1)粒子所受电场力的大小。

(2)粒子的运动区间。

(3)粒子的运动周期。

答案

解：(1)由图可知，x=0与x=d(或-d)两点间的电势差为φ0

电场强度的大小

电场力的大小

(2)设粒子在[- x0，x0]区间内运动，速率为v，由题意得：

 ①

由图可知 ②

由①②得： ③

因动能非负，有：，得

即 ④

粒子的运动区间为：

(3)考虑粒子从- x0处开始运动的四分之一周期

根据牛顿第二定律得，粒子的加速度 ⑤

由匀加速直线运动

将④⑤代入，得

粒子的运动周期⑥

选择题

在根尖的四个结构中，哪个结构细胞停止伸长，已经分化（　　）

A．成熟区

B．伸长区

C．分生区

D．根冠
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单项选择题

Just how does a person arrive at an idea of the kind of person that he is He develops this (1) of self through a gradual and complicated (2) which continues throughout life. The notion is an (3) that one establishes only with the help of others. (4) the elementary knowledge that one is short or tall is a comparative judgment that we cannot make (5) we have the opportunity to (6) ourselves with others. One’s idea of qualities which are harder to (7) is even more dependent upon other people’s ideas.

Whether one is intelligent, or stupid; attractive, or ugly; these and many other ideas of the self are (8) from the reactions of people with whom we (9) This process of (10) the nature of the self from the reaction of others has been labeled the "Looking-glass Self" by Cooley, who carefully analyzed this psychological (11) of self-discovery.

Just as the picture in the mirror gives an image of the physical self, (12) the perception of the reactions of others gives an image of the social self. We know, (13) , that we are good at certain things and not at others. This (14) came to us from the reactions of other persons, first our parents and then other individuals (15) in life. It should be remembered that, as other people’s reactions could be (16) and understood in more than one way, the looking-glass self with which the individual (17) may easily differ from the image others have actually formed of his (18) Clearly, it is our perception of the responses of others and not their (19) responses that (20) our self-image, and these perceptions are often not accurate.

20（）

A.improves

B.shapes

C.directs

D.guides
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