How to Escape from a Car Window (SLOW MOTION) - Smarter Every Day 144

Hey, it's me Destin. Welcome back to Smarter Every Day. Have you ever been driving along and you suddenly stop and realize that you're moving around this world in a bubble of glass? It's kind of weird if you think about it. But it's really cool.

Engineers have designed the glass in your car to be extremely safe, so that when you have a wreck you decrease the chance that you're getting cut in half like the bad guy in Ghost. Remember the movie Ghost? Anyway, so today on Smarter Every Day I want to investigate the two different types of glass that go into car windows. On the side, you have tempered glass, kind of like a Prince Rupert's Drop, but flat. And on the windshield, you have annealed glass.

First, let's talk about windshields. Glass on a windshield is designed to take a hit from a rock and not fall in on your lap. However, glass on the side windows is different. To demonstrate that, I got a window out of a car in a junkyard and we're gonna hit it and just watch what happens. Check out how it breaks. It doesn't break. That's because it's designed to be very, very strong in the center but not on the edge. Watch what happens if we do the same thing on the edge. [shatters] The whole thing shatters.

To understand exactly why this happens, we're gonna go to my driveway and use a high-speed camera to analyze this at 20,000 frames per second. What are you doing Mr. Helper?

• Holding glass.

• He's holding the glass. This is not tempered glass. We've got a high-speed camera, this is a V1610 which at the time that we made this video is the fastest high-speed camera on the market. And I have another helper over here. Not you. What are you doing?

• Pushing the button when the camera breaks.

• I mean when the, um..

• The camera's not gonna break! [laughs]

• The glass.

• When the glass breaks, and what does that do?

• Start the camera.

• That's right. OK. So what we're gonna do is we're gonna drop tempered glass and not tempered glass, and we're gonna show the difference between the two. And there's a special announcement at the end of this. So, let's do some science.

You can move buddy.

• I can move?

• I've got it. Yep, safety glasses on. You ready little girl? Ready? 3.. 2.. 1.. [shatters] [laughs] OK, did you do it? [slowed shatter] OK you ready buddy? Tempered. Are you ready? Do your best. 3.. 2.. 1.. Did you get it? It didn't break. The tempered glass didn't break. Dropping from higher this time. Try to drop it on the corner too. 3.. 2.. 1.. Yahaa! It's not breaking! OK so it wasn't breaking so we're gonna tape this wrench onto the bottom of the glass. And the idea is that maybe the wrench will actually twist the edge and break the glass when it impacts. We'll see. Hopefully it works. 3.. 2.. 1.. [smashes] There we go. [slowed smash]

OK this is the aftermath. You can see on the left we have tempered glass. On the right, that's just normal annealed glass so the question is, why do they break differently? It's pretty awesome though isn't it. OK so what did we learn from this experiment?

Annealed glass is just normal glass that's been heated up in the manufacturing process and then allowed to cool down gradually. This cooling relieves any internal stresses on the glass. You'll notice when the annealed glass breaks, not only is the cracking slow but sometimes it doesn't even make it to the other side of the glass. Which is good for windshields. A small crack in one spot of the windshield doesn't completely blind your vision. Also, they put a piece of plastic between two sheets of annealed glass so that these big shards don't fall in and cut you inside the car.

Tempered glass is very different. Instead of allowing the glass to cool down slowly, they use forced air to cool down the outer sides of the glass really quickly, which leaves the inside of the glass hot. Kind of like how a Prince Rupert's Drop is made if you remember that video. The inside is still hot so as it cools down, it shrinks and pulls in which puts the outside of the glass in compression but the inside in really high tension. Just like the Prince Rupert's Drop, if you can get through that hard outer layer, it releases the tension and the feathering front destroys the entire sheet of glass.

On the Prince Rupert's Drop, you have to get through the outside curved surface of the drop to release the tension. But on a car window, you have to get through the flat tempered surface to release the tension. With a sheet of glass though, we have to start it along one of the edges. Even after you break tempered glass, you can sometimes still hear the stress releasing itself as it continues to break.

So there you go. The next time you need to escape from a car window, remember that the windshield is annealed and it's gonna hold together but the side windows are tempered and they're gonna bust out completely as long as you hit them on the edge.

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I want to say thank you to a really awesome security guard that decided not to stop me. Thank you very much. What was your name?

• Tony.

• Tony. Tony has common sense and he's awesome. I really appreciate that.

kiwicrate.com/smarter. First link in the description. Bye.