利用圖中的滑輪組裝成最省力的滑輪組用來提升重物，請畫出繞繩方法。

（2）如圖所示，用裁紙刀裁紙.加在裁紙刀上的動力為F，支點在O點，請在圖中畫出動力臂并用L標明．
，

試題分析：一只定滑輪和一只動滑輪組成的滑輪組，最省力的繞繩結果就是由3段繩子承擔物重，按要求依次繞繩連接即可．如圖，由3段繩子承擔物重，是最省力的繞繩方法

題中告知了杠桿的支點和動力F的示意圖，而動力臂是支點到動力的作用線的距離，因此過支點做力的作用下的垂線段即可．首先作力F的作用線，然后過支點O作力F作用線的垂線段，標出力臂的符號，答圖如下：

點評：在滑輪組繞繩之前，先要明確其由幾段繩子承擔物重，才按一定的要求依次繞繩．本題考查了力臂的畫法．畫力臂時關鍵是要畫出支點到力的作用線的距離．
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