Falling Arches and Keystone Science

Hi –

This week in Science, we learned about keystones and catenary arches.

First, we checked out these two signs:

Keep an eye out for them the next time you go on a road trip (which is why we are doing this lesson this week). Pennsylvania turnpike signs are shaped like keystones (Pennsylvania is called ‘The Keystone State’)…but what’s a keystone? Hmmm…

We switched our attention to a river. Let’s say that while I was driving my car, I came to a river and I wanted to get to the other side. What could I do?

​Swim?

How about driving across on a bridge?

That would probably be a better idea – especially if I wanted to use my car on the other side of the river…

but what if my car was very heavy – as heavy as this elephant?


The bridge squished. Now what?

Build a stronger bridge.

Like this?


No problem.

What’s the difference between the bridges?

Obviously, the second bridge was a lot stronger than the first. Since they are made of the same material, the only real difference was the shape. The first was rectangular underneath while the second was rounded.

The second bridge had more support than the first and could hold up the elephant.
​Moving on, we took a look at a ‘necklace.’ If you hold it at both ends, the shape it forms is called a ‘catenary.’

If you turn that shape upside-down, it’s called a catenary arch.

A catenary arch is a very strong shape and it looks a lot like the shape under the stronger bridge.

The piece in the very center at the top of the arch is called the keystone. It is colored pink on the bridge below.

Scientific Note: Catenary arches are useful because any weight placed on top of the keystone is distributed over the entire structure (that way it won’t fail in the middle like our first bridge could). That’s the reason that many bridges are built on top of one or more arches.

We looked at some pictures of bridges with keystones. In the photos, they are colored pink. In real life, they are usually the same color as the rest of the arch. 🙂

If you’ve ever taken Washington Blvd. to the car wash or the zoo, you’ve passed under this bridge:

​The next time you’re out that way, take a good look at the keystones!

Next, we watched how to make a catenary arch out of cardboard pieces.


​OK – so our small arch was interesting (and the elephant liked it), but we wanted to build something a bit bigger.

This time, the children put the arch together by themselves – with just a little help.

​Ta Da…

That arch was fun – but we wanted to crawl through it and it wasn’t quite big enough. What if we built an arch large enough for us to walk under? We definitely wanted to try that!

Now we are professional arch builders!

We all took turns walking under the arch.

Morah Chen

almost made it through the arch without knocking it over…

but not quite.

That’s ok – it was a good try.

Great teamwork everybody!

Happy Thanksgiving – I’ll see you next week,

Morah Elaine

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