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有一质量为m=2.0×103kg，发动机的额定输出功率P0=40KW，行驶在平直公路上时所受阻力恒为车重的0.1倍．若汽车从静止开始先匀加速启动，加速度的大小a=1m/s2．达到额定输出功率后，汽车保持功率不变又加速行驶了一段时间，直到获得最大速度后才匀速行驶．试求：

（1）汽车的最大行驶速度vm；

（2）汽车匀加速运动的时间t；

（3）当速度为6m/s时，汽车牵引力的瞬时功率．

答案

（1）当阻力等于牵引力时，汽车达到最大速度为vm=

P额
F
=
P额
f
=
40000
2000
m/s=20m/s

（2）根据牛顿第二定律得：

F-f=ma

F=ma+f=4000N

匀加速运动的最大速度为：v=

P额
F
=
40000
4000
m/s=10m/s

t=

△v
a
=
10-0
1
s=10s

（3）当汽车速度是6m/s时，汽车处于匀加速阶段，

所以P=Fv=4000×6W=24000W

答：（1）汽车的最大行驶速度为20m/s；

（2）汽车匀加速运动的时间t为10s；

（3）当速度为6m/s时，汽车牵引力的瞬时功率为24KW．

连线题

请将下列动物与其“本领”用线连起来。

(1)啄木鸟A．田野卫士
(2)猫头鹰B．森林医生
(3)喜鹊C．捕鱼能手
(4)黄鼠狼D．灭鼠益兽
(5)鸬鹚E．捕鼠能手
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问答题

In the coming decades, Europe’s influence on affairs beyond its borders will be sharply limited, and it is in other regions, not Europe, that the 21st century will be most clearly forged and defined. Certainly, one reason for NATO’ s increasing marginalization stems from the behavior of its European members. With NATO, critical decisions are still made nationally; much of the talk about a common defense policy remains just that--talk. There is little specialization or coordination. Missing as well are many of the logistical and intelligence assets needed to project military force on distant battlefields. With the Cold War and the Soviet threat a distant memory, there is little political willingness, on a country-by-country basis, to provide adequate public funds to the military.
Political and demographic changes within Europe, as well as the United States, also ensure that the transatlantic alliance will lose prominence. In Europe, the E. U. project still consumes the attention of many, but for others, especially those in southern Europe facing unsustainable fiscal shortfalls, domestic economic turmoil takes precedence. No doubt, Europe’s security challenges are geographically, politically and psychologically less immediate to the population than its economic ones. Mounting financial problems and the imperative to cut deficits are sure to limit what Europeans can do militarily beyond their continent. It is true that the era in which Europe and transatlantic relations dominated U.S. foreign policy is over.
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