WarmGlass.com

I caught wind of a few videos like this, where a small piece of ceramic thrown with not much force shatters the side-windows of an automobile.

http://www.youtube.com/watch?v=Vyw60MMC ... re=related

Can anyone help me understand the physics? When I break tempered glass, I generally need to tap it on edge with something hard, but I have literally hammered the flat face with no visible effect, so this video is surprising.

When you strike the flat surface of tempered glass with a hammer, the force of the blow is spread out across a broader area so that it doesn't shatter. If the hammer blow is at an angle or if the hammer head is very small or pointed, rather than flat, you can break tempered glass.

With a spark plug or similar small item, all of the force of the throw is directed to one small spot, rather than spread out. That's what causes the window to break. Porcelain is really hard and if a broken piece of spark plug is used you have very small pointed edges which make it easier to break the window.

As an aside, windshields (windscreens) are usually made of laminated glass (two layers with a plastic film between), while car windows are usually made of tempered glass. Using tempered glass allows a trapped occupant to break the glass from the inside. You can even buy hammers with a hardened steel point for the purpose of breaking the glass in an emergency. http://www.lifehammer.com/

To simplify what Brad said
Tempered glass:
baseball : bounce Sharp point : shatter

The edge is more vulnerable than the surface. A hammer blow works fine on the edge. Another tool that will shatter the surface is an automatic center punch. This is a steel point in a spring loaded device. http://www.harborfreight.com/spring-loa ... h-621.html

I certainly knew where to ask this question.

That spring loaded center punch is sold in Florida as an emergency escape tool if you should happen to drive your car into one of the many canals. It happens fairly often.

Jim