题目内容
一辆汽车在平直的公路上以速度v0开始加速行驶,经过一段时间t,前进了距离l,此时恰好达到其最大速度vm,设此过程中汽车发动机始终以额定功率P工作,汽车所受的阻力恒为f,则在这段时间里,发动机所做的功为( )
①fvmt ②Pt ③
mvm2-
mvo2+f·l ④ft![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824190801245557.png)
①fvmt ②Pt ③
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824190800621338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824190800621338.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824190801245557.png)
A.①② | B.②③ | C.①③④ | D.①②③ |
D
试题分析:汽车从速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824190801604323.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824190802009367.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241908024151075.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241908027421038.png)
当然,由于发动机功率恒定,则经过时间t,发动机所做的功也可以为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824190803132527.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824190803538742.png)
故选D
点评:由题意可知,求发动机做的功,当功率恒定时,可以由功率与时间的乘积,也可以由动能定理求出.当功率不恒定时,必须由动能定理求出.
![](http://thumb2018.1010pic.com/images/loading.gif)
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