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Feeling imbalanced? Now you'll get to work on that wobble as you create the least organic forest of all time!

I made this project to start investigations in to balance, levers, center of mass, and recycling with my students. After seeing their wild arboreal creations, it has already become one of my absolute favorite activities. No two trees are alike, they're pretty (as far as plastic trees go), and the most careful of science explorations is interrupted by highly entertaining *THUMPS!* as trees come tumbling down.

  • What: Balancing Bottle Trees!
  • Concepts: balance, center of gravity, levers, torque
  • Cost: ~$5 for solo building, ~$30 for a classroom
  • Time: ~45 minutes to make

If you want to learn more about torque and balance, Mythbusters has a great quick writeup on the physics of seesaws, which harkens all the way back 1800 years ago to Archimedes' Law of the Lever. As you start to construct these trees, you'll get to live these principles in real time.

Let's go!

Step 1: Stuff You'll Need!

The supplies are simple and easily available! For this project, I used:

  • 1.5" PVC (choose your amount)
  • 1.5" PVC connectors (T-joint, Elbow, and 45 degree)
  • Recycled Bottles

The PVC can be found at most hardware stores, and you can change the size if you want.

For the bottles, I found quite a bit of them in, well, the recycling dumpster which meant they needed a bit of scrubbing to get rid of the smell. But hey, science is messy. :)

Step 2: Cut Your PVC Branches

First up is to cut the branches that your bottles will hang on. I used a bandsaw and a jigsaw, but any will do. The most important cut on several of the ends is to make an angled cut which allows for the initial balancing angle. I found that a 30 degree cut worked well (so that the pipe stands at 60 degrees) for getting the balance just right.

For the branches, you can cut them to different lengths for fun, and choose whether you want angled or straight cuts on the different ends. Straight cuts will allow branches to be connected more easily but not be the initial balancer, so it's a tradeoff!

NOTE: If you want to do the next step (hole drilling) before you cut the branches, that works fine, too!

Step 3: Drill Bottle Holding Holes

Using a 1 1/4" hole saw on a drill press, I made holes along my bottle branches. I drilled them in lines with different spacings so that there were more variables to play with. You could even drill holes on both sides of the pipe if you want!

The one important note is that on your pieces that you cut at an angle, make sure to drill holes on the same side as the longer side of the PVC so that the bottles can counterbalance the tilt of the PVC. If that seems confusing, you'll get what this means once you start balancing bottles.

NOTE: When cutting PVC, you want to drill at slow speeds and use the "peck" method of going up and down to let your material cool. If you start melting the PVC with the drill, you're applying too much pressure and it will volatize some of the nastiness in PVC.

Step 4: Fill 'Em Up!

Water time! I took off the labels (for aesthetic), filled up my bottles partway, and capped them. It's great to keep a container and a funnel nearby for when you're building. With these, you can easily pour out water and re-fill bottles to change the weight.

Step 5: Assemble a Balancing Bottle Forest

You are ready to balance! Oh there's just so much to do! You can make small or big structures, in two dimensions or three. Balance 2-liter bottles with tiny ones, make them wide or narrow. You'll find that bottle caps kind of notch into the holes naturally, and can be left there. I was amazed and surprised at how stable some of these structures ended up being. Don't let a few tumbles get you down, as there are so many more things to build!

Here are a couple challenges that you can use in the classroom or at home:

  • How tall can you make it?
  • How wide can you make it?
  • How many bottles can you balance?
  • How high up can you balance a single bottle?
  • Can you use the T-joint?
  • Can you make it balance in multiple dimensions? As in go 3D like in the last photo?
  • Can you balance a 2-liter bottle on one side against your smallest bottle on the other
  • Can you balance a bottle not using one of the PVC holes?

Have fun, keep exploring, and I'd love to learn what you discover! Comment below!

<p>Have you thought about using this to explain how a low center of balance is more stable than a high center of balance? This would make a good visual for heavy bottles down low making for a more stable &quot;tree.&quot;</p>
<p>This looks like fun :)</p>

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Bio: The Oakland Toy Lab is a community-based wonder lab for students to build, tinker, explore, make, break, and learn! We are writing up engaging science ... More »
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