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.
(republished on 07/11/06)