如图所示，一质量为m、电量为q的小球在电场强度为E的匀强电场中_爱下问搜题

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问题多选题

如图所示，一质量为m、电量为q的小球在电场强度为E的匀强电场中，以初速度υ₀沿直线ON做匀变速运动，直线ON与水平面的夹角为30°．若小球在初始位置的电势能为零，重力加速度为g，且mg=Eq，则（　　）

A．电场方向竖直向上

B．小球运动的加速度大小为g

C．小球最大高度为

V

20

2g

D．小球电势能的最大值为

MV	20
4

答案

A、因为小球做匀变速直线运动，则小球所受的合力与速度方向在同一条直线上，结合平行四边形定则知，电场力的方向与水平方向夹角为30°，斜向上．如图所示．故A错误．

B、根据平行四边形定则知，小球所受的重力和电场力相等，两个力的夹角为120°，所以合力大小与分力大小相等，等于mg，根据牛顿第二定律知，小球的加速度为g．故B正确．

C、小球斜向上做匀减速直线运动，匀减速直线运动的位移s=

v02

2a

=

v02

2g

，则小球上升的最大高度h=

1

2

s=

v02

4g

．故C错误．

D、在整个过程中电场力做功W=qEscos120°=-

1

4

mv02，电势能增加

1

4

mv02，所以小球电势能的最大值为

1

4

mv02．故D正确．

故选：BD．

完形填空

-I was six when I joined my father and two elder brothers at sunrise in the fields of Eufaula, Okla. ___31__ the time I was eight I was helping Dad fix up old furniture. He gave me a cent for every nail I__32__out of old boards.
　　I got my first__33__job, at JM’s Restaurant in town, when I was 12. My main responsibilities(职责) were__34__tables and washing dishes,__35__sometimes I helped cook.
　　Every day after school I would__36__to JM’s and work until ten. Even on Saturdays I__37__from two until eleven. At that age it was difficult going to work and__38__my friends run off to swim or play. I didn’t necessarily like work, but I loved what working__39__me to have. Because of my__40__I was always the one buying when my friends and I went to the local Tastee Freez. This made me__41__.
　　Word that I was trustworthy and hard-working__42__around town. A local clothing store offered me credit (赊帐)__43__I was only in seventh grade. I immediately__44__a $68 sports coat and a $22 pair of shoes. I was__45__only 65 cents an hour, and I already owed the storekeeper $90! So I learned__46__the danger of easy credit. I paid it__47__as soon as I could.
My first job taught me self-control, responsibility and brought me a__48__of personal satisfaction few of my friends had experienced. As my father,__49__worked three jobs, once told me, "If you__50__sacrifice(奉献) and responsibility, there are not many things in life you can’t have." How right he was.

单项选择题

一产妇西医诊断为破伤风，中医应诊断为

A．产后血晕
B．产后血劳
C．产后痉病
D．产后发热
E．产后乳汁自出