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问题 计算题

如图所示，物体从光滑斜面上的A点由静止开始下滑，经过B点后进入水平面，最后停在C点，现每隔0.2 s通过传感器测量物体的运动速率v，下表给出了部分测量数据，设物体经过B点前后速率不变，重力加速度g取10 m/s2，求：

（1）物体在AB段和BC段的加速度a1和a2；

（2）物体与水平面之间的动摩擦因数μ；

（3）物体运动到B点时的速率VB。

答案

解：（1）由公式可得两段的加速度大小分别为：

a1=5 m/s2，方向沿斜面向下；a2=2m/s2，方向水平向左

（2）根据牛顿第二定律：-μmg=ma2

代人数值得μ=0.2

（3）由表可知，物体开始运动的时刻t0=0.2s，设运动到B的时刻为t，则有如下运动学方程：

VB=a1（t－t0）

v1.4=VB+a2（t1.4－t）

由以上二式解得：t=0.7s，VB=2.5m/s

单项选择题

Until recently, most population growth has occurred in relatively dense urban environments. In (91) words, even if we were witnessing rapid population growth, that would not mean that the (92) were filling in with people. (93) , the urban areas would tend to become more crowded. Historically, we find that people have moved to (94) the jobs were. Jobs were usually in ports or locations with many natural (95) . After transportation became relatively cheap, other geographical locations became (96) of economic activity. One factor that determines where people will move to, even when transportation becomes cheap, is (97) . And individuals working in cities (98) higher incomes than those working in (99) areas. Hence, population has tended until the 1970s to (100) in our major urban centers. Recently population has (101) out of the Northeast to the South and to the West for the more pleasant climates. If people did not (102) from living in large cities, there would likely be a much more even population distribution (103) our entire land area. Probably, if this (104) the case, there would be less concern over population (105) Indeed, one need only (106) a cross-country drive or plane trip to realize how (107) populated these United States really are. This is not to say that we should or should not do something about (108) growth. The shortage of people in certain areas merely demonstrates that overpopulation in the United States is really only a problem in (109) urban environments. And this problem may be slowly correcting (110) .

A．population

B．education

C．people

D．culture
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单项选择题 A1/A2型题

辛味药物的主要作用是（）

A.行气、活血

B.收敛、固涩

C.补益、缓急

D.软坚、散结

E.燥湿、通泄
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