题目内容
某人站在高楼的平台边缘处,以
="20" m/s的初速度竖直向上抛出一石子。求抛出后,石子经过距抛出点15 m处所需的时间。(不计空气阻力,g取10
)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153641204.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153673251.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153688643.gif)
当石块在抛出点上方距抛出点15m处时,取向上为正方向,则位移
,代入公式
,得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153766688.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241141537822215.jpg)
化简得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153797528.gif)
解得:
=1s,
=3s。
=1s对应看石块上升时到达“离抛出点15 m处”时所用的时间,而
=" 3" s则时应着从最高点往回落时第二次经过“离抛出点15m处”时所用的时间。
由于石块上升的最大高度H="20" m,所以石块落到抛出点下方“离抛出点15 m处”时,自由下落的总高度为
,下落此段距离所用的时间
。这样石块从抛出点到第三次经过“离抛出点15m处”时所用的时间为
。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153704663.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153751536.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153766688.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241141537822215.jpg)
化简得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153797528.gif)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153829199.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153860201.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153829199.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153860201.gif)
由于石块上升的最大高度H="20" m,所以石块落到抛出点下方“离抛出点15 m处”时,自由下落的总高度为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153907655.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153922948.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824114153688643.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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