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问题 选择题

如图所示，物体A和B的质量分别为2kg和1kg，用跨过定滑轮的细线相连，静止地叠放在傾角为θ=30°的光滑斜面上，A与B间的动摩擦因数为

3
5
，现有一平行于斜面向下的力F作用在物体B上，设最大静摩擦力等于滑动摩擦力，若要使物体运动，则F至少为（g=10m/s2）．（　　）

A．20N

B．22N

C．11N

D．17N

答案

A受重力、支持力、摩擦力和绳子的拉力，如图，

根据正交分解得：

mAgsin30°+μmAgcos30°=T

解得：T=16N

对整体分析，受力如图，根据正交分解得：

（mA+mB）gsin30°+F=2T

解得：F=17N．

故D正确，A、B、C错误．

故选D．

单项选择题

Lichens may grow on the bark of a tree in a steaming tropical rain forest, on the bricks of big city buildings, on rocks in hot springs, on wind-swept mountaintops, and in the driest desserts. In the arctic, they provide the principal food for caribou, and they are one of the few plants that grow in Antarctica. They are pioneers, appearing in barren rocky areas and starting the formation of soil in which mosses, then ferns, and then other plants can take root.
Lichens are a partnership of two plants — fungi and algae. The lichen body is made up of a network of fungal strands. In the upper layers of these grow groups of algae. The two organisms live together to the benefit of both, a relationship known as symbiosis. The fungi provide support, absorb water, and shelter the tender algae from direct sunlight. The algae carry on photosynthesis and provide the fungi with food. The algae can live independently and are recognizable as a species that grows alone. The fungi, on the other hand, cannot live apart from their partners. They can be placed in known classes of fungi but are unlike any species that lives independently.
So definite are the form, color, and characteristics of these double organisms that for hundreds of years, they were classified as one. More than 15,000 "species" were named. If these organisms are classified as separate species, it is difficult to fit them into the existing system of classification. But if they are classified separately, these species of fungi seem rather strange. Lichens are a splendid example of the difficulties faced by taxonomists in classifying species.

The word "splendid" underlined in Paragraph 3 most probably means ______ .

A．unique
B．improbable
C．excellent
D．famous
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问答题 简答题

自我介绍时，要注意哪些问题？
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