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

V20
2g

D.小球电势能的最大值为

MV20
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.
小题1:
A.BeforeB.WithinC.FromD.By
小题2:
A.pulledB.putC.pickedD.pressed
小题3:
A.usualB.realC.mainD.particular
小题4:
A.sweepingB.packingC.clearingD.emptying
小题5:
A.orB.soC.butD.even
小题6:
A.headB.turnC.changeD.move
小题7:
A.studiedB.workedC.playedD.slept
小题8:
A.helpingB.havingC.watchingD.letting
小题9:
A.askedB.toldC.promisedD.allowed
小题10:
A.studyB.powerC.ageD.job
小题11: 
A.proudB.friendlyC.luckyD.hopeful
小题12: 
A.ranB.gotC.flewD.carried
小题13:
A.althoughB.whileC.ifD.since
小题14: 
A.soldB.borrowedC.chargedD.wore
小题15:
A.keepingB.makingC.payingD.taking
小题16:
A.graduallyB.greatlyC.hardlyD.early
小题17:
A.outB.overC.awayD.off
小题18:
A.pointB.levelC.partD.sign
小题19: 
A.heB.thatC.whoD.whoever
小题20:
A.understandB.demandC.offerD.fear
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