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问题 选择题

一物块静止在粗糙的水平桌面上．从某时刻开始，物块受到一方向不变的水平拉力作用．假设物块与桌面间的最大静摩擦力等于滑动摩擦力．以a表示物块的加速度大小，F表示水平拉力的大小．能正确描述F与a之间的关系的图象是（　　）

A．

B．

C．

D．

答案

物块受力分析如图所示：

由牛顿第二定律得；F-μmg=ma

解得：F=ma+μmg

F与a成一次函数关系，故ABD错误，C正确，

故选C．

单项选择题

设R是一个2元关系，S是一个3元关系，则下列运算中正确的是（）。

A．R-S

B．R×S

C．R∩S

D．R∪S
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填空题

According to traditional Cherokee beliefs, we are part of (1) . We don’t (2) with it and we’re not trying to (3) it. We are a part of the Great Life. And within the Great Life, there are three great Laws of Nature, which tell us how we have to live in (4) with everything else. The First Law of Nature is that you don’t take any life without real (5) . So we shouldn’t (6) needlessly. We believe everything is alive, including animals, plants and (7) . And so to us, taking the life of a plant is just as grave a 8() as taking the life of an animal. The Second Law is that everything we do should (9) the Great Life. And everything we do (10) the Great Life, and everything that (11) within the Great Life affects us. So it’s very important that what we do will not (12) other parts of the Great Life. The Third Law basically is that we don’t (13) where we live. And where we live is not our (14) , small community. It’s this (15) . We don’t pour chemical (16) down the drain because they all wind up in the water. It might seem a little difficult to live by those three laws in this (17) society. But there are still a lot of things that we can do to bring these laws into our lives, such as (18) , using the (19) as little as possible, and (20) gardening.
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