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When you think of the circus, you probably think of things like acrobats and trapeze artists and tightrope walkers. You probably don’t think a lot about science. But every single circus act I just named makes use of the science of physics, the study of how matter and energy move.
There are so many acts in the circus that require years of diligent training, excellent physical conditioning, and expensive specialized equipment. There are a few, though, that you can at least get a feel for in the comfort of your own home. Today’s experiment involves some stuff you probably have lying around the kitchen. We’re going to spin some plates!
Materials:
- Paper plates
- A dowel rod about 3 feet long
- Tape
- Coins
- An adult
Process:
- To spin plates properly, you need a stable pole. This is where your adult comes in. Their job, for as long as you want to spin the plates, is to hold your dowel rod straight up for you with one end on the floor. Don’t let them sneak away to do grown-up stuff. It’s science time.
- Tape two or three paper plates together. You’re doing this so your spinning plate has a little weight. Often paper plates are so light a weak breath will blow them away.
- Place the center of your plate on the top of your dowel rod and spin away! It will probably fall off pretty quickly the first time. Try, try again until you can get the plate balanced and spinning for a few seconds at a time. If you’re really careful, you can keep spinning it at the edge while it’s moving. If you’re having trouble, try adding another paper plate or two to bulk it up.
- Once you’ve got the hang of it a bit, it’s time to turn this plate spinning into an experiment. Tape a coin to the plate, somewhere near the outer edge. Center your plate on the dowel rod and try spinning again. What happens differently this time?
- Try to think of ways to rebalance your plate. Could you add more coins? What if you tried spinning it from somewhere other than the center? Can you get your coin/plate combo to spin like the plate alone did?
- Try adding new coins in different places on the plate. What happens when you put one dead center? What happens when you put some in between the center and the outer edge? Why do you think these changes make a difference to the plate’s ability to spin?
Summary
What we are experimenting here with something called center of mass. An object’s center of mass is the point where all of its mass is evenly spread out around that point. Finding an object’s center of mass allows you to balance it. When you’re trying to balance on one foot, part of what makes it hard is that you have to find your own center of mass, but since you have appendages that can move around, your personal center of mass is constantly changing.
A normal plate’s center of mass is right in the center of the plate. That’s why, with some practice, you can get it spinning and balanced on the dowel rod. When you tape a coin near the edge, you’re adding mass to the plate, and since it’s not balanced, the plate’s center of mass moves over towards the coin. You may have found that if you tried to spin the coin/plate combo by putting the dowel rod closer to the coin it worked a bit better. You also may have found that adding another, equal mass coin to the opposite end of the plate also helped out.
What other ways could you change the plate’s center of mass?
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