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Ripping a gigantic, thousand-page phone book in half sounds like a feat that’s reserved only for the strongest people. But with the help of a bit of physics, this giant artifact from a bygone age can crumble.

Well, tear. The idea is to use properties that all materials have in common to weaken the book until it gives way. And that tells us something about how things fail.

The trick is to create a deep crease in the book. This creates something called a stress concentrator. Stress concentrators are like pressure points that experience more force than other parts of an object.

They work because of geometrical discontinuities like corners and cracks, which change up how force is distributed in an object, basically concentrating it at that point. Sharp corners or creases experience a lot more stress than, say, a flat part in the middle of an object. We pay a lot of attention to stress concentrators in the design of everyday things.

They’re the reason why perforated paper is easier to tear, and why some food packages have a little cut to help you open them without scissors. They’re even why airplane windows are oval instead of square, so there aren’t any corners that might fail under pressure. One classic example that illustrates the effects of stress concentrators involves a rectangular plate that’s being pulled at the same rate from the top and bottom.

If the plate doesn’t have any cracks or holes or other imperfections, it stretches out uniformly. But if you cut a small notch in the plate, the force will be amplified around it, especially near the tip. And the deeper or sharper that tip is, the more intense that force becomes.

Which means more and more force is concentrated in a small area, and the whole thing gives way at the site of that notch. But even if you’ve prepped your phonebook with the sharpest V you’ve ever made, having a stress concentrator won’t magically make you rip a phonebook in half. For each part of the phonebook you rip, you still need enough force to exceed the tensile strength and tear strength of the pages you’re tearing.

The tensile strength describes the force you need to create a rip in the paper, while the tear strength tells you how easy it is to keep tearing those pages once you’ve ripped them. Pulling on the paper causes the fibers within the paper to separate or even break, which in turn lets you rip, and keep ripping, the paper. Minimizing the number of pages you’re tearing at one time also helps, since fewer pages require less force to break.

The deep indentation helps with that, since it both concentrates the force you’re applying, and means you’re only working on a few pages at a time. With some effort, it should be possible to get through an entire phone book this way. Stress concentration doesn’t only apply to small stuff like phone books and cheese packages.

Structures like bridges, which have to carry a lot of weight, can give way prematurely if stress concentrators are present. Which can be as a result of flaws in the overall design or just poor construction. We’re not saying you should try tearing a phone book in half at your next party.

But we’re also not saying you shouldn’t. If you can even find a phone book these days, that is. Thanks for asking!

And if you want to ask us a question like this that might get made into an episode, you can get access to our QQ inbox by becoming a patron over at patreon.com/scishow. And that also helps us continue making educational videos for everyone to enjoy, so we really appreciate it! [♪ OUTRO].