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Q & A: Glass vs. Paper

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Most recent answer: 10/22/2007
When I pull hard at opposite ends of a piece of paper it rips into two pieces. But when I drop a glass bottle on the sidewalk it breaks into a hundred pieces. Why the difference?
- Richard Treptow
Chicago State University

Glass is a very hard substance (in other words, it does not bend when you put forces on it), whereas paper is flexible. If you bend a piece of paper it will not break, it just folds. If you try to bend a piece of glass nothing happens unless you use a lot of force, in which case the glass will shatter.

When you apply a force to one end of a piece of glass, the entire piece feels this force. The way to see this is to imagine that you are holding on to one end of a glass rod. If your friend grabs the other end and pulls or pushes or twists, you will feel this at your end.

Paper is different. If you unroll some toilet paper and hold on to one end and your friend the other end, your friend can twist and push on the paper all day and you will not feel anything at your end. The only thing that you will feel is if your friend pulls on the paper like a rope, although if she pulls too hard the paper will rip.

The paper will start to rip at the weakest point. Once it starts to rip, this point becomes even weaker, so it continues to rip at about the same place until it is torn in half.

With glass, something else happens. Since a force on one part of the glass is felt by the whole piece, if the force is large enough to break the glass it can break in several places since the force is felt everywhere. When you drop a glass bottle on the ground a very large force is produced at the point where the glass hits the ground. This force is felt "all over" the bottle, so breaks it "all over".

Thanks for the excellent question!

~Ann & Mats

PS: It turns out that when you drop a bottle on the ground, it often doesn't break when it hits the ground but rather when it starts to bounce up. This is because the structure of glass can handle being compressed (when it hits the ground), but it is much weaker for forces that pull it apart. When the glass is bouncing up, it has forces that are acting to pull it apart.

(published on 10/22/2007)

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