问题 计算题

如图所示,在距水平地面高h=0.80 m的水平桌面一端的边缘放置一个质量m=0.80 kg的木块B,桌面的另一端有一块质量M=1.0 kg的木块A以初速度v0= 4.0 m/s开始向着木块B滑动,经过时间t=0.80 s与B发生碰撞,碰后两木块都落到地面上,木块B离开桌面后落到地面上的D点。设两木块均可以看做质点,它们的碰撞时间极短,且已知D点距桌面边缘的水平距离s=0.60 m,木块A与桌面间的动摩擦因数μ=0. 25,重力加速度取g=10 m/s2。求:

(1)两木块碰撞前瞬间,木块A的速度大小;

(2)木块B离开桌面时的速度大小;

(3)木块A落到地面上的位置与D点之间的距离。

答案

解:(1)木块A在桌面上受到滑动摩擦力作用做匀减速运动,根据牛顿第二定律,木块A的加速度

=2.5 m/s2

设两木块碰撞前A的速度大小为v,根据运动学公式,得v=v0-at=2.0 m/s

(2)两木块离开桌面后均做平抛运动,设木块B离开桌面时的速度大小为v2,在空中飞行的时间为t',根据平抛运动规律有

解得

(3)设两木块碰撞后木块A的速度大小为v1,根据动量守恒定律有:Mv=Mv1+mv2

解得

设木块A落到地面过程的水平位移为s',根据平抛规律

则木块A落到地面上的位置与D点之间的距离△s=s -s'=0.28 m

完形填空

第二部分:语言知识及应用(共两节,满分35分)

第一节:完形填空(共10小题;每小题2分,满分20分)

阅读下面短文,掌握其大意,然后从16--25各题所给的四个选项(A、B、C和D)中选出最佳选项,并在答题卡相应的16--25上将该项涂黑。

In 1956 Phoenix, Arizona, was a city with boundless blue skies. One day as I walked around the house with my sister Kathy’s new parakeet (小鹦鹉)on my finger, I wanted to show Perky 16_____ the sky looked like. Maybe he could make a little bird friend out there. I took him into the backyard, and then, to my 17______, Perky flew off. The enormous(巨大的), blue sky swallowed up my sister’s blue  treasure and suddenly he had gone, clipped wings and all.

Kathy managed to forgive me. With fake optimism(乐观), she even tried to reassure (安慰)me that Perky would find a new  18 ______. But I was far too clever to 19______ that such a thing was possible.

Decades later, I watched my own 20_______ growing. We shared their activities, spending soccer Saturdays in folding chairs with the parents of the kids’ friends, the Kissells. The two families went camping around Arizona together. We became the best of friends. One evening, the game was to tell Great Pet stories. One person claimed(宣称) to 21______ the oldest living goldfish. Someone else had a psychic dog. Then Barry, the father of the other family, took the floor and 22 ________ that the Greatest Pet of All Time was his blue parakeet, Sweetie Pie.

"The best thing 23______ Sweetie Pie," he said, "was the way we got him. One day, when I was about eight, out of the clear, blue sky, a little blue parakeet just floated down and landed on my finger."

When I was finally able to 24_______ , we examined the amazing evidence(证据). The dates and the locations and the pictures of the bird all matched up. It seems our two families had been connected long before we ever met. Forty years later, I ran to my sister and said, "You were 25_______! Perky lived!"

16. A. what     B. how    C. which D. where

17. A. joy       B. horror      C. disappointment  D. satisfaction

18. A. parent   B. home  C. master D. life

19. A. imagine       B. suppose      C. doubt  D. believe

20. A. birds    B. happiness   C. worries      D. children

21. A. catch    B. find    C. buy     D. have

22. A. announced   B. said    C. told    D. hoped

23. A. in        B. about  C. of       D. on

24. A. think    B. speak  C. interrupt     D. explain

25. A. right     B. wrong C. silly    D. mad

单项选择题