问题 问答题

如图所示,质量mA为4.0kg的木板A放在水平面C上,木板与水平面间的动摩擦因数μ为0.24,木板右端放着质量mB为1.0kg的小物块B(视为质点),它们均处于静止状态.木板突然受到水平向右的12N?s的瞬时冲量I作用开始运动,当小物块滑离木板时,木板的动能EM为8.0J,小物块的动能为0.50J,重力加速度取10m/s2,求

(1)瞬时冲量作用结束时木板的速度v0

(2)木板的长度L.

答案

解;(1)设水平向右为正方向,有:I=mAv0

代入数据得:v=3.0m/s                            ②

(2)设A对B、B对A、C对A的滑动摩擦力的大小分别为FAB、FBA、FCA

B在A上滑行的时间为t,B离开A时A和B的速度分别为vA和vB,有-(FBA+FCA)t=mvA-mAv0

FABt=mBvB

其中FAB=FBA FCA=μ(mA+mC)g     ⑤

设A、B相对于C的位移大小分别为sA和sB,有-(FBA+FCA)sA=

1
2
mvA2-
1
2
mv02

FABsB=EKB

动量和动能之间的关系为:mAvA=

2mAEKA

mAvA=

2mBEKB
   ⑨

木板A的长度 L=sA-sB

代入数据解得:L=0.50m

答:(1)瞬时冲量作用结束时木板的速度为3.0m/s;

(2)木板的长度L为0.50m.

完形填空
A mouse looked through a crack(裂缝) in the wall to see the farmer and his wife opening a package. “What food might this contain?”The mouse____.He was alarmed to discover that it was a mousetrap(捕鼠夹子).
Going back to the farmyard the mouse gave a(n)____:“There is a mousetrap!” The chicken raised her head and said: “Mr Mouse, I can tell this is a big____to you, but it is of no consequence to me.I cannot be____by it.”
The mouse  turned  to  the  pig. “I  am  so  sorry,”sympathized the pig, “but there is____I can do about it but____.I will make sure that you are in my prayers.”
The mouse turned to the cow.“Mr Mouse, I'm sorry for you,____it's no skin off my nose,” said she.
So he returned to the house, head down and___, to face the farmer's mousetrap alone.
That very night a sound was heard throughout the house,like the sound of a mousetrap____its prey.The farmer's wife rushed to see what was caught. In the darkness, she didn't see that it was a____ snake whose tail the trap had caught.The snake bit her.
The farmer rushed her to the hospital.She returned home with a fever.Now everyone knows you____a fever with fresh chicken soup, so the farmer took a knife to the____for the chicken.
His wife's sickness___so that friends and neighbors came to sit with her around the clock.To feed them the farmer killed the pig.
The farmer's wife didn't get____and died.So many people came for her____that the farmer had the____ killed so as to____meat for all of them to eat.
The mouse looked upon it all from his____in the wall with great____.
So, the next time you hear someone is facing a problem and think it doesn't concern you, remember—when one of us is threatened, we are all at____.
小题1:
A.whispered B.shouted
C.wondered D.hesitated
小题2:
A.warning B.message
C.noticeD.information
小题3:
A.surprise B.concern
C.resultD.deal
小题4:
A.bothered B.cheated
C.murdered D.exchanged
小题5:.
A.anything B.something
C.everythingD.nothing
小题6:
A.wait B.pray
C.avoid D.prevent
小题7:
A.soB.and
C.but D.for
小题8:
A.astonishedB.puzzled
C.depressed D.satisfied
小题9:
A.catching B.protecting
C.gathering D.collecting
小题10:.
A.poisonous B.nervous
C.fat D.big
小题11:
A.checkB.treat
C.abandon D.examine
小题12:
A.room B.farmyard
C.kitchen D.wall
小题13:
A.appeared B.reduced
C.disappeared D.continued
小题14:
A.good B.safe
C.quiet D.well
小题15:
A.funeral B.event
C.farm D.death
小题16:
A.chicken B.cow
C.snake D.pig
小题17:
A.offerB.afford
C.provide D.supply
小题18:
A.houseB.position
C.crack D.expression
小题19:
A.happinessB.sadness
C.tiredness D.carelessness
小题20:
A.peaceB.risk
C.sight D.work
解答题