點解鉛筆插入去有水ge保鮮袋唔會流水!!?

其實E個現像係因為圓筆頭是尖的,當插入保鮮袋時,筆尖只是插了一個小洞,但鉛筆卻以高速插入,閉塞了保鮮袋的洞,水便不能流出來.

The plastic bag will not burst when poking a pencil in it is due to the stress at the hole did not reach the “yield stress”. An inflated plastic bag with water did not have a high internal pressure that would cause the edge of the hole continue to failure and become the catastrophic failure (fast fracture) stage.

But when you poking into an inflated balloon are totally different, since the high internal pressure cause the edge of the hole experience a fast fracture. The balloon will burst immediately. However, if you stick some adhesive paper on the balloon before poking with a needle, the balloon will not burst.

You can try it yourself with a balloon.

For details, please refer to material science book or search on the web for the topic of “fracture mechanic”.

2008-11-25 11:24:22 補充：
個保鮮袋唔爆係因為個洞周圍未到“yield stress”. 因為個袋o既內部壓力未足夠導致個洞周圍產生快速的斷裂階段(fast fracture).

但用氣球就唔同, 一捅就爆, 因為個氣球o既內部壓力非常大, 導致個洞周圍產生快速的斷裂.
如果貼D膠紙係個氣球面, 就唔會爆(因為yield stress 加大左) 你可以自己試o下.

你可以參考”material science” D書, 個topic係 “fracture mechanic”.

原理*