Homeschool STEM: Drinking Straw and Twine Suspension Bridge

I know that I've been boring lately and just posting our homeschool work and school-related tutes and not, like, thoughtful essays about life and parenthood and whatever, but frankly, I'm actively avoiding introspection until my emotions feel a little less gut-shot, soooo...

Here's another tute!

The kids and I have been having a fine time, the past couple of months, working on a California unit study that I created for them. They (or I) pick something in California that they'd like to see or do, and then I create a week-long unit based on that thing. We won't have time to get to everything, of course, just as their grandparents won't have time to take them to everything when they're actually in California, but it's fun for the kids to anticipate all of the adventures that they'll have, and I've been pleased at what a lovely cross-curricular study this has naturally become, especially when it fills in some subjects that we haven't studied previously, like coastal geography, the Gold Rush, or this week's unit on bridge engineering in honor of the Golden Gate Bridge, which the kids will certainly see (many times) on their trip, and which their grandmother has already promised that they can actually walk across, as both kids deeply desire to do.

To understand bridge engineering, you need to understand the forces of compression and tension. We discussed these forces, and I showed the kids some diagrams, but I could see from looking at their blank little faces that it really was not sinking in. Good thing, then, that my plans also included building a model of every single bridge type that we discussed!

I'll show you the models in more detail, but let me just say now that as soon as the kids made their models of each bridge design and tested them, they immediately, naturally, understood how compression and tension work. You could see the very moment that it came alive from looking at their bright little faces. THAT is why I'm so obsessed with hands-on, project-based learning!

To understand why a suspension bridge works so well, you should first explain a beam bridge, and then build one and test it to the point of catastrophic failure. The way that you test it will undoubtedly not resemble the way that a real bridge experiences live weight (on account of you don't really hang a giant hook attached to a giant bucket filled with giant pennies from a real bridge...), but it's adequate for testing and comparing a bridge's strength, and examining its points of failure.

The kids built beam and arch bridges out of building blocks, but also built them out of drinking straws, using the tutorial in this Scientific American article. Even though you can make beam bridges in endless ways, for the purpose of comparing its effectiveness to the suspension bridge, you want to have the kids make it just the way the article says:

Most of my photos of this project are crap, because apparently I don't know how to work my camera anymore.

I had to help the kids with this a little--it's something I don't usually like to do with their projects, but it was that important in order to make the comparison work.

Our bucket was also heavier than the Styrofoam cup the article calls for, but as cool as this project is, it cannot make me waste both five drinking straws AND a Styrofoam cup!

Have the kids gently drop pennies into the cup, one at a time, until the bridge fails:

Our beam bridge held 31 cents before it reached catastrophic failure. Note where and how it fails and how many pennies it held. If I'd been thinking, I'd have made the kids take notes and draw diagrams... oh, well!

Next, build the suspension bridge. You'll see that it's built almost exactly the way the beam bridge is built--

--except with the addition of that twine:

That twine, my Friends, is what is going to make all the difference!

Doesn't the completed suspension bridge look elegant?

Test the bridge the same way that you tested the beam bridge. Don't tell the kids in advance, but it will hold more--WAY more--money than the puny beam bridge did. We're talking $2.53 worth of suspension bridge awesomeness!



That's because the main cable pulls, or provides tension, in the opposite direction from the beam's compression, and that force is balanced by the cable's placement over the tower and anchored on the other side. The cable pulls the beam up, the tower takes the weight, and the force is balanced on both sides.

I'm not ashamed to tell you that before this unit, I didn't understand how suspension bridges work (I was also in my twenties before I could reliably spell "refrigerator," so there's also that...). Like the kids, I've seen the Golden Gate Bridge tons of times, and the way that the cable drapes, as if it's simply resting there, always threw me off. But it turns out--and you can model this, as well!--that the cable is held that way because it's not just attached to the deck in the middle, but by suspender cables all the way across. That drape is simply the way that the cable looks when it's got all of those points of attachment.

We had a playdate with friends instead of most of our school yesterday, but we're stuck home today with the second car in the shop because the Lord doesn't want us to have any money, so hopefully we'll have time for both our bridge-building challenge and the computer modeling that I wanted to do to finish out this study.

And those kids better take TONS of pictures of themselves on the Golden Gate Bridge for me!