Ninja rocks

[Wolfman]

Okay, I'm just hoping that someone here can give me an explanation of how/why this works. Got sent a link to a video that shows how throwing a large brick at a car window doesn't even crack it, but throwing just a small piece of ceramic from a spark plug makes it shatter. Thought it was just another trick at first, but then checked it out on Wikipedia, and found out that these so-called "ninja rocks" are actually classified in some places as burglary tools, and are frequently used in smash-and-grab types of auto theft.

So, it seems to be legit, but I'm not sure why. It can't just be an issue of density or hardness...if I threw a diamond of the same size or weight at a car window, I doubt it would cause it to shatter. I assume there must be something more to it than this, but the Wikipedia entry just explains what it is...not how it works.

Comments, anyone?

 

[Freethinker]

A wheel bearing (not the carrier, the individual bearing) will easily break a car window. I'd expect it is the small contact point with the glass and the relatively high density of the object impacting it, compared to the larger contact point and relatively low density of the brick. A 120lb woman stepping on your toes in high heels is painful compared to a 200lb man in rubber-soled shoes.

 

[tsg]

When I was a firefighter, we were doing extrication drills when our captain showed us how hard it was to break a car window with a hammer, and how easy it was to break it with something pointy, especially in the corner.

I have no idea how or why it works, except that I once heard someone say that tempered glass is structurally under a lot of pressure and causing a very small fracture can make it shatter.

 

[jimbob]

tsg,

look at a windscreen through some polaroid sunglasses, and if you are lucky, you should see the rainbow colours almost like stress contours, and supporting your statement.

Nature doesn't like sharp corners.

IIRC: the outside of the tempered glass is compressing the inside, so a slight crack, and pouf, the inside almost "springs" out.

 

[Kilgore Trout]

People, or at least the one guy that told me about this trick, were using this as early as the late 80s. He also said that an important part of it is that it's the broken parts of the ceramic -- a whole spark plug won't do it. He did not call it a "ninja rock" however.

 

[Wolfman]

A wheel bearing (not the carrier, the individual bearing) will easily break a car window. I'd expect it is the small contact point with the glass and the relatively high density of the object impacting it, compared to the larger contact point and relatively low density of the brick. A 120lb woman stepping on your toes in high heels is painful compared to a 200lb man in rubber-soled shoes.

I'm quite familiar with the ball bearing idea...but that usually involves holding the ball bearing in your hand, and slamming it against the window, does it not? Or can you also cause the window to break by simply throwing the ball bearing at the window?

In the video in question, someone simply throws this tiny bit of spark plug at the window, and it breaks...that's the part that raises my question. Were they hitting the window while holding it in their hand, I'd have no such problem.

 

[Freethinker]

I'm quite familiar with the ball bearing idea...but that usually involves holding the ball bearing in your hand, and slamming it against the window, does it not? Or can you also cause the window to break by simply throwing the ball bearing at the window?

I'll try to describe the method I'm familiar with: A right-handed person would hold the bearing in his right palm with his right side toward the car. The right hand is pulled up onto the left shoulder and then slung quickly toward the window and the bearing released. The window just disappears. It was primarily used to steal women's purses at traffic lights. It happens so quickly they don't even know what's happening.

 

[DavidS]

<undocumented engineer's speculative opinion follows>

Glass is fairly hard. Bricks are only kinda hard, not especially sharp, and generally more brittle and friable than glass. Hit a window with a brick, and the corner of the brick is likely to fail first and spread the impact over enough area to avoid inititating enough crack in the glass.

Intact sparkplugs are hard on the ceramic part, but kinda smooth all over and the metal parts that hit first are on the malleable side. Toss a sparkplug at the window and the smooth ceramic parts may slide enough, or the malleable metal parts deform enough, to spread the load and not crack the glass.

Bearing races are hard. Rap a window soundly with a bearing and it won't deform enough to spread the load. The concentrated load cracks the glass.

A broken sparkplug has hard, sharp ceramic corners exposed. Toss one into a window and the corners can "dig in" to the glass without deforming, keeping the impact concentrated on an area small enough to crack the glass.


Glass is brittle, especially if it's bearing the residual stresses of tempering. What "brittle" means is that stress concentrations at sharp corners -- like the edge of a crack -- can easily and rapidly propagate the crack, and the resulting motion of the strain released by the crack further elevates the stress at the crack tip. Of course, none of this comes into play until a sufficiently long and sharp crack gets initiated. Brittle also means "sufficiently" needn't be "very" long. Once you get a decent crack started in tempered glass, the residual stresses can be enough to propagate cracks through the whole pane.

Stresses concentrate at sharp corners like crack tips, which is why in more malleable materials existing cracks are easier to extend than initiating new ones. That's why that tiny little crack from a gravel ding can grow (with vibration or thermal cycling) across your entire (untempered but laminated) windshield. That's why good technique for repairing cracks often involves drilling a hole at the tip -- to increase the radius of curvature there to spread the stress concentration out.

 

[JoeEllison]

I saw this one on Mythbusters, I think...

 

[IXP]

A couple of days ago I saw a CNN item about how to get out of a car if you go in the water. Apparently, it can be very hard to open a window. As a last result, they suggested you keep a center punch in the glove box, and if all else fails, you could just tap the windshield with the point and it would shatter.

IXP

 

[tsg]

A couple of days ago I saw a CNN item about how to get out of a car if you go in the water. Apparently, it can be very hard to open a window. As a last result, they suggested you keep a center punch in the glove box, and if all else fails, you could just tap the windshield with the point and it would shatter.

IXP

Technical correction: not the windshield, but the window in the door. Windshields typically have a plastic film between two panes of glass to keep them from shattering.

You'll also have better luck breaking the window in the corner rather than in the center where it won't deflect as much.

 

[Wolfman]

Thanks, all