问题 问答题

如图所示,木板长L=1.6m质量M=4.0kg,上表面光滑,下表面与地面的动摩擦因数为0.4,质量m=1.0kg的小滑块(视为质点)放在木板的右端,开始时木板与物块静止,现给木板向右的初速度,(g取10m/s2)求:

(1)木板所受摩擦力大小

(2)使小滑块不从木板上掉下,木板初速的最大值.

答案

(1)木板与地面间压力大小等于N=(M+m)g…①

故木板所受摩擦力Ff=μ(M+m)g=20N…②

(2)木板的加速度a=

Ff
M
=5m/s2…③

滑块静止不动,只要木板位移小于木板的长度,滑块就不掉下来,根据v02=2aL得,

v0=

2aL
=4m/s

即木板初速度的最大值是4m/s.

答:(1)木板所受摩擦力大小为20N.

(2)使小滑块不从木板上掉下,木板初速的最大值为4m/s.

阅读理解

阅读理解。

     When I was growing up, I was embarrassed to be seen with my father. He was badly crippled (跛脚),

and when we would walk together, his hand on my arm for balance, people would stare and I would be

ashamed of the unwanted attention. If ever noticed or bothered, he never let on.

     It was difficult to walk together-and because of that, we didn't say much as we went along. But as we

started out, he always said, "You set the pace. I will try to follow you."

     Our usual walk was to or from the subway, which was how he got to work. He went to work even in

bad weather. He almost never missed a day, and would make it to the office even if others could not. It was

a matter of pride for him.

     When snow or ice was on the ground, it was impossible for him to walk, even with help...Such times

my sister or I would pull him through the streets of Brooklyn, N. Y., on a child's sleigh to the subway entrance.

Once there, he would try to grasp handrail until he reached the lower steps that the warmer tunnel air kept

ice free. In Manhattan the subway station was the basement of his office building, and he would not have to

go outside again until we met him in Brooklyn on his way home.

     When I think of it now, I am surprised at how much courage it must have taken for a grown man to suffer

from shame and disability. And I am also surprised at how he did it-without bitterness or complaint.

     He never talked about himself as an object of pity, nor did he show any envy of the more fortunate or able. What he looked for in others was a "good heart", and if he found one, the owner was good enough for him.

     Now that I am older, I believe that is a proper standard by which to judge people, even though I still don't

know exactly what a "good heart" is. But I know the times I don't have one myself.

     He has been away for many years now, but I think of him often. I wonder if he sensed my reluctance to

be seen with him during our walks. If he did, I am sorry I never told him how sorry I was, how unworthy I

was, how I regretted it. I think of him when I com- plain about my troubles, when I am envious of another's

good for-tune, when I don't have a "good heart".

1. How did the man treat his father when he was young?

A. He helped his father happily.

B. He never helped his father.

C. He helped his father, but not very happily.

D. He only helped his father take a walk after supper.

2. As a disabled man, his father ____. 

A. didn't work very hard

B. didn't go to work from time to time

C. hated those who had good fortune

D. was happy and satisfied, and never lost hope

3.What does the underlined word "reluctance" mean in the article? It means ____.

A. anger

B. sadness

C. happiness

D. unwillingness

4. How did the father get to work usually?

A. By subway.

B. By bus.

C. By wheelchair.

D. By bike.

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