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如图所示在车厢中有一条光滑的带子（质量不计），带子中放上一个圆柱体，车子静止时带子两边的夹角∠ACB=90°，若车厢以加速度a=7.5m/s2向左匀加速运动，则带子的两边与车厢顶面夹角分别为多少？

 

答案

设车静止时AC长为，当小车以向左作匀加速运动时，由于AC、BC之间的类似于“滑轮”，故受到的拉力相等，设为FT，圆柱体所受到的合力为ma，在向左作匀加速，运动中AC长为，BC长为

由几何关系得

由牛顿运动定律建立方程：

代入数据求得

单项选择题

下列关于中央税的叙述中不正确的是( )。

A．中央税属于中央财政的固定收入

B．其管理权属于中央，但使用权不完全属于中央

C．通常把维护国家权益和有利于进行宏观调控的税种作为中央税

D．现行中央税包括关税、消费税、银行总行营业税，铁道营业税等
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填空题

When I was growing up, I was embarrassed to be seen with my father. He (1) severely crippled and very short, and when we would walk together, his hand on my arm (2) balance, people would stare. I would inwardly be afraid (3) squirm at the unwanted attention. If he ever noticed or was bothered, he never let (4) .
It was difficult to coordinate (5) steps - his halting, mine impatient - and because of that, we didn’t say much as we went along. But as we started out, he always said, "You (6) the pace. I will try to adjust to you. "
Our usual walk was to or (7) the subway, which was how he got to (8) . He went to work sick, and (9) nasty weather. He almost never missed a day, and would (10) it to the office even if (11) could not: a matter of pride!
He never talked about himself as an object of pity, (12) 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 (13) which to judge people, even though I still don’t know precisely (14) a "good heart" is. But I know the times I don’t have one myself.
He has (15) gone many years now, but I think (16) him often. I wonder if he sensed my reluctance to be seen with him during our (17) . If he did, I am (18) I never told him how sorry I was, how unworthy I was, how I regretted it. I think of him when I complain about trifles, when I am (19) of another’s good fortune, when I don’t have a "good heart".
At such times I put my hand on his arm to regain my (20) , and say, "You set the pace, I will try to adjust to you.\
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