Physics of leaning back in a chair and building bridges

Date:19 May 2017 Tags:, ,

What does leaning back in a chair have in common with building bridges? Physics!

By David Grossman

One thing about physics: everywhere you go on the surface of the Earth, it’s pretty much the same. What’s surprising is how many connections there are. For example, the same rules that apply to keeping bridges from falling down also nicely explain what happens when you’re leaning back in your chair, trying to maintain balance.

Practical Engineering shows how the two are related.

Our understanding of these concepts goes back pretty far—all the way to Isaac Newton in 1687, when he published Principia Mathematica in which he discussed his famous three laws of motion. The second one applies here, which says that a net force on an object will cause it to accelerate. The goal here is to get something to stop moving: keep a bridge up, keep your chair stable. To keep things static, forces have to be balanced.

The goal of civil engineering is often to achieve zero force, to balance out all stresses on an object until it remains still. So when you’re leaning back on a chair, you’re trying to achieve that same balance.

In a chair-sitting scenario, your feet are great at compression. They can push you back pretty easily. However, they’re terrible at creating tension, defined here as pulling forward. As your centre of gravity shifts in a lean, your body eventually moves past a point of rotation in the chair. Your feet have no way of creating tension and you fall down—not in the middle of a class or meeting, we hope.

If getting to a point of balance on a chair is this difficult, trying to achieve no motion on a public project like a bridge takes even more work. They’re also among the most needed infrastructure projects in America. So once you’re able to maintain a perfect balance, it might be time to start considering engineering school.

Image credit: Getty

Video credit: Practical Engineering



This article was originally written for and published by Popular Mechanics USA.

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