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Science & Technology

Science Experiment Sheds Light On Planet Orbits

That's the sound of a penny rolling round and round in a device called a gravity funnel. This was recorded at the Natural Science Center in Greensboro using WUNC's Make Radio iPhone app. It's part of a collaboration with the North Carolina Science Festival. You've likely seen this game, the coin launches from the top of a big round funnel and then spins, on edge, down to the collection jar underneath the hole at the bottom of the cone.

To sort out the science behind why this works, we've invited Richard Superfine in to talk us through this sound. He's a professor of Physics and Astronomy at UNC Chapel Hill. Rich thanks for joining us.

Richard Superfine: It's great to be here, Eric.

Rich, explain to us what we are hearing?

Richard Superdine: So I think the first impression you have is this mesmerizing, repetitive sound of the coin circulating around the center hole of the funnel. I think that's most easily understood in the sense of planets going around the sun in kind of the inexorable motion of a planet.

But unlike planets, these are collapsing inwards, right? They're coming in toward the center.

Richard Superfine: That's right. So as the coin gets launched and starts rolling around the funnel, it has asserted enough velocity to maintain a stable orbit around the middle just like planets have stable orbits around the sun. But they lose energy to air, to rolling resistance. And as they do that, they lose their energy and slide further down the funnel.

Our friend Jonathon Frederick from the North Carolina Science Festival recorded this sound for us with his iPhone. After he tried the penny, he upped the stakes and tossed in a nickel. This sounds different. Can you tell us why?

What do you think?

Richard Superfine: There's different ways in which the coin can lose energy. For example, it can wobble and lose energy due to the wobbling and I think the way Jonathon launched this nickel, it must have gone down the chute very smoothly. It's an amazing sound hearing how gradually it moves in, you think of the stable orbits that the coin is taking around the middle.

We've got one more for you, it's a penny racing a dime down to the bottom of the gravity funnel.

What are you thinking when you listen to these two coins race to the bottom? The both collided at the top but maintained their orbit.

Richard Superfine: So as long as they have a velocity they will have an orbit and if you take two coins that have different sizes and they have different weights.

Explain how that's more like maybe a satellite than a planet in the way that their orbits are decaying really.

Richard Superfine: So satellites going around the earth will lose energy with the resistance of the atmosphere and in losing that energy, they eventually can spiral in and hit the Earth. Other systems which also lose energy, that we know of, are black holes circulating around each other. So you can think of that coin going down that black hole of the funnel like two black holes circulating around each other. The way black holes lose energy is gravitational waves and that's one of the hottest topics right now on Astrophysics is trying to detect those gravitational waves.

So this very simple experiment can take you all the way to the latest black hole research?

Richard Superfine: It can take you from astrophysics to the banked track at the NASCAR Charlotte Motor Speedway.

Which works in the same way.

Richard Superfine: That's right.

I'd like to try that next. Rich, thanks for joining us.

Richard Superfine: It's great to be here, Eric.

Richard Superfine is a professor of Physics and Astronomy at UNC Chapel Hill. The sounds we heard were recorded with WUNC's new Make Radio iPhone app at a North Carolina Science Festival event in Greensboro. The Festival runs through Sunday at locations across the state.

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