A Ball Dropped Through The Earth Becomes A Permanent Pendulum

What happens when you drop a ball down a hole drilled through the center of the Earth? The answer might surprise you.

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Occasionally, when it's a slow news day, we wind up with holes in the show; gaps of a minute or two that the news of the day doesn't quite fill.

NPR science correspondent Joe Palca offered to fill those with science stories about holes.

JOE PALCA, BYLINE: Today, my hole story is about what happens when you drop a ball down a hole. Well, it falls to the bottom, of course. But what if the hole goes right through the center of the Earth and comes out on the other side? That's a question freshman physics students are often asked to answer at the California Institute of Technology.

Do you really ask freshman physics students to do this?

DAVID STEVENSON: Yes.

PALCA: David Stevenson is a professor of Planetary Science at Caltech. He offered to help make sure I get this right.

Now, before purists start screaming that such a hole would be impossible, chill out - this is a theoretical problem. And to make things simple we'll assume our Earth is all made of the same kind of rock, there's no friction in the hole, and our fake Earth isn't rotating. So what happens?

Well, as the ball drops through the hole it picks up speed - that's the acceleration due to gravity. It reaches its top speed at the very center of the globe and continues rocketing toward the other side. Now here's the tricky part - because gravity is tugging on it, as it heads for the other side of the world, it starts to slow down, at first a little bit, and then more as it approaches the surface.

And just as the ball reaches the surface it comes to a complete stop, but just for a moment. Then it falls back down the hole and the process repeats over and over and over, like a kind of intra-planetary pendulum but with no strings attached.

So that's my answer. OK, how was that?

STEVENSON: Sounds great.

PALCA: Whew, that's lucky. I barely passed freshman physics.

Well, thank you Dave Stevenson, professor of planetary science at Caltech.

Joe Palca, NPR News.

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