The best thing about high speed cameras is they reveal things you couldn't see before. Even watching recordings of ordinary things offers extraordinary results—in this case, the progression of cracking glass as it shatters. It's pretty cool.

In the video above you can see the Slow Mo Guys record shattering Pyrex at a 343,915 frames per second. For comparison, typical cameras record at 30 or 60 fps. Of course, if you played the video at the same frame rate it was recorded, you wouldn't see anything. The solution is to play it back at a normal speed of 30 fps or so.

So what can we do with this video? How about using it to measure the speed at which the shattering propagates through the glass? Yes, let's do that.

Video Analysis of Crack Speed

When you have a video, especially one from the Internet, you don't always know all the details. If we want to look at the speed of an object in the video, there are three related properties:

The frame rate

The scale—the size of the object in the frame

The speed of the object in the frame

If you know two of these things, you can find the third. For this video, we know the frame rate (the Slow Mo Guys are kind enough to include it in the video). To find the speed of the crack, I must first find the scale. Fortunately, the Slow Mo Guys used a common object—a Pyrex measuring glass. It seems that I found one just like it. Here you can see that from the tip of the handle to the inner curve is about 0.074 meters.

Now for the video analysis. Once I have set the scale in the video and changed the frame rate (to the real frame rate), I can mark the location of the front edge of the crack as it moves down the handle. You could use a few different video analysis programs, but I like Tracker Video Analysis (it's free).

The first part of this data corresponds to the leading edge of the crack moving along the curve of the handle such that the velocity along the y-axis would not be constant. However, if you look only at the straight part of the handle, you realize the crack's position changes uniformly with time. By fitting a linear function to this data, I find that it has a speed of 417 m/s (932 mph). Just to be clear, that is super fast. That is faster than the speed of sound in air (340 m/s) but not faster than the speed of sound in glass (4540 m/s). Actually, HyperPhysics lists the speed of sound in Pyrex at 5640 m/s.

But why is the speed of sound greater in a solid than in air? Here is my super-short answer. Sound is a moving disturbance in a medium. In air, some particles interact with nearby particles, causing them to move. Then these moving particles push other particles, and so on. The same thing happens in a solid, but with one big difference—density. Since the density of a solid is so much higher than a gas, the distance between particles is much smaller. This allows the disturbance to propagate faster and makes the speed of sound higher.

One more thing. If you are a big fan of video analysis and physics, you might want to check out my recent book—*Physics and Video Analysis. *If you are an IOPScience subscriber, you can access the electronic version of the book at IOP Science.