Why did the glass shatter?

Question

My friends and I were out and sitting at a round glass table. One of my friends emphatically made a statement while pounding his mug on the table. It was kinda hard but not THAT hard....not hard enought where anyone would think the glass would break...yet the glass completely shattered. We noticed he hit the table exactly in the middle and the shatter pattern was circular all the way out. Just wondering if there is some physics explanation as to why the glass would shatter easier if you hit it exactly in the middle?

oh, sorry i guess i left out....it wasnt just a glass table with nothing under it. it was a round metal table with a circular glass over it (maybe like 1/3 inch thick). so it wasnt really like breaking a board in the middle.....unless the board is flat against a wall.

no, it was just ONE pound. =) but yeah, i noticed the circular pattern and they did seem spaced the same distance. thats why i thought of some type of wave or resonance.....but dont know enough about physics to explain why it would break so easily.

Answer 1

That's an interesting question. The table was made of "tempered" safety glass, just like car windows.

Here's how tempered glass works: after being cast, the surface of the glass plate is rapidly heated and then cooled. This creates a large amount of strain on the surface, in such a way that normal bending and pressure actually relieves the strain, rather than increasing it, thus making the glass much stronger.

But since the mug was made of ceramic, which is harder than glass, it was able to make a tiny chip or crack in the glass. The enormous strain in the glass was then able to widen the crack, and cause it to propagate and zip though the entire plate at more than supersonic speed. Tempered glass is strong enough, that vibration was probably not an issue, but the fact that the mug was ceramic and harder than the glass was.

It's actually supposed to break into small pieces, since if it were normal glass it might otherwise break into a few heavy, big, razor sharp chunks, and might have sliced into your knees!

This would be even worse if it was a car window breaking during a crash, sending big glass shards flying, instead of tiny pebble sized chunks. In the 1930's and 40's, people sometimes bled to death from lacerations caused by flying glass during an accident.

You can make tempered car window break by throwing small chips of porcelain (e.g; broken spark plug insulators) at them...
http://www.youtube.com/watch?v=y8tx3B5YxNQ
....just be sure to do this with junk car windows, unless you are an antisocial jerk and enjoy seriously pissing other people off.

Hope that answers your question....
~Donkey Hotei

Answer 2

Though there probably was some weird type of resonance, I think that your friend hit hard enough, and far enough away from the metal support edge, that the mechanical advantage was enough to break it. It's the same idea of having a hammer with a short handle versus one with a long handle, except that the leverage is on the point receiving the transferred energy instead of the one imparting it.

Answer 3

You say "was pounding" his mug. Does that mean repeatedly? If so, it's possible that this action set up harmonic resonance in the table (if he was pounding at the resonant frequency for the table's glass). This may well have concentrated the vibrational energy at the centre of the table, causing it to shatter there. Are the concentric rings regularly spaced?

Answer 4

Glass is not very flexible. As you experienced, it will shatter if you try to "bend" it. It is easier to get a piece of glass to "bend" and thus shatter the farther you are away from where it is supported (the edge of the table).

Just think of the deflection of a long piece of wood. Is it easier to bend deflect (bend) it in the middle or hear one of its supported ends.

It has to do with the mechaical advantage of levers.

Answer 5

That IS the weakest spot. It'd be like breaking a board- It snaps a lot easier if you stomp the middle vs. the side. It'd be interesting to see the pattern though, I wonder if the break was fractal.