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Duration:02:33
Uploaded:2017-06-20
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You may have seen rock sculptures seemingly defying physics in your newsfeed, but what's actually happening?

Hosted by: Olivia Gordon
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Sources:

https://books.google.com/books/about/Physics.html?id=LHk8OAAACAAJ
http://hyperphysics.phy-astr.gsu.edu/hbase/cm.html
https://www.khanacademy.org/science/physics/linear-momentum/center-of-mass/a/what-is-center-of-mass
https://www.grc.nasa.gov/www/k-12/airplane/cg.html
http://wonderfulengineering.com/heres-the-secret-behind-how-this-guy-can-balance-rocks-in-any-arrangement/
(Intro)

You've probably seen them making the rounds on social media sites every so often: giant stacks of rocks, usually with some of the more enormous rocks stacked on top of smaller ones and sticking out at odd angles.  Some of them look almost impossible, like they're violating the laws of physics just by existing but theoretically, you can balance on any rock on pretty much any part of its surface.  You maybe need to take advantage of a couple of physics principles: center of gravity and counterbalance.  You also need a lot of patience.

Something's center of gravity is basically the spot where there's the same amount of weight on all sides.  In other words, where its weight balances out.  So whenever you're trying to balance something, what you're really trying to do is find its center of gravity.  For something to stay balanced, its center of gravity has to be above what's known as the area of contact.  Essentially, it has to be directly over the place where it touches the ground, or if there's more than one point where it touches the ground, like a chair with four legs, the center of gravity has to be over the area you'd outline by connecting all of the points of contact, and that's the key to rock balancing.

Say your stack of rocks has one small rock at the bottom and you're trying to balance a bigger rock on top.  To get the bigger rock to stay, you just need to position it so that its center of gravity is above the smaller rock, the area of contact.  Easier said than done when you're talking about a huge unwieldy rock, though, since shifting its position by the tiniest bit will change its center of gravity by a lot.  This is also where counterbalancing comes in.

If you want to have one rock stick out at a weird angle, that rock's center of gravity might not be above the area of contact, but you can position a second rock on the other side so that the center of gravity of both rocks combined is above the area of contact.  That way, they'll balance each other out.

So to become an expert rock stacker, you just have to keep these principles in mind.  You'll want to look for places on the rock where there are at least three close points of contact with the surface you're balancing it on.  That'll widen the area of contact that you'll need to balance the center of gravity over.  Then, you just have to be patient as you try to get the center of gravity in the right spot.  Luckily, rocks are pretty easy to come by, so if you wanna experiment with art and physics, you can try your hand at making a rock balancing sculpture of your own.

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