题目内容
某人用F="50" N、竖直向下的恒力拉绳的一端,使绳绕过一定滑轮(不计滑轮的摩擦和质量)拉一物体沿粗糙水平面由A移到B,如图7-1-9所示,已知定滑轮离物体的高度H="3" m,物体所受阻力恒为22 N,求拉力和阻力对物体各做多少功.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241204551023677.jpg)
图7-1-9
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241204551023677.jpg)
图7-1-9
-76 J.
拉力F="50" N,方向竖直向下,是恒力,其作用点在绳端,当物体从A点沿水平地面移至B点的过程中,绳端(力的作用点)下移距离为
,则人对绳的拉力F所做的功为:
.
人通过细绳(质量不计)拉物体运动,因为不计滑轮质量及摩擦,人对绳的拉力所做的功与绳对物体的拉力(虽大小等于F="50" N,但方向在变,是变力)所做的功相等,所以绳的拉力对物体所做的功等于WF="127" J.
物体从A移至B的过程中,所受地面阻力为F′="22" N,为一恒力,位移为
则阻力对物体所做的功为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120455180624.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120455195901.gif)
=-22×3×(1.73-0.577)J="-76" J.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120455117710.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120455133914.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120455149839.gif)
人通过细绳(质量不计)拉物体运动,因为不计滑轮质量及摩擦,人对绳的拉力所做的功与绳对物体的拉力(虽大小等于F="50" N,但方向在变,是变力)所做的功相等,所以绳的拉力对物体所做的功等于WF="127" J.
物体从A移至B的过程中,所受地面阻力为F′="22" N,为一恒力,位移为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120455164795.gif)
则阻力对物体所做的功为:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120455180624.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824120455195901.gif)
=-22×3×(1.73-0.577)J="-76" J.
![](http://thumb2018.1010pic.com/images/loading.gif)
练习册系列答案
相关题目