Why does glass break more easily than plastic. It seems like it should break less easily than plastic because it's stronger than plastic.
[Physics] Why is glass so breakable
material-science
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As far as I remember, there is some truth behind this statement: glass is inherently extremely strong, but it is fragile in practice because of microcracks on its surface. In water, glass dissolves to some extent. As a result, microcracks partly disappear, and glass becomes much stronger (at least for a while). I remember reading that A. F. Ioffe (I guess his name was sometimes written as Joffe) held a thin glass stick in water for a few days, and the stick became flexible. Maybe I'll try to find a reference later.
EDIT: the following article confirms that holding glass in water can make the glass stronger, but the authors used hot water:
Stockdale, G. F., Tooley, F. V. and Ying, C. W. Changes in the Tensile Strength of Glass Caused by Water Immersion Treatment. Journal of the American Ceramic Society, vol. 34, no. 4, p.116, 1951. doi: 10.1111/j.1151-2916.1951.tb11618.x
Suppose you bend a perfect, i.e. unscratched, piece of glass, the forces on it look like:
The top of the glass is in tension and the bottom in compression, but the stress is spread over a large area of glass so the local stress at any point isn't enough to break the glass.
Now put a scratch in the top surface and bend it again:
This time the stress is concentrated at the leading edge of the scratch so you get a high local stress and the glass breaks at the tip of the scratch. This means the crack grows until it breaks through the glass. That's why putting a scratch on the glass causes it to break when bent. The scratch focusses the stress at the scratch.
Best Answer
Glass is brittle because it has many microscopic cracks in it which act as seeds for a fracture. If you can make glass without these cracks, as is done in fiberglass, then it is not so fragile.