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Falling Freely

Week of August 17, 1998
It has been said (well, by me mostly) that all science is really just physics. Astronomy is no exception; without physics, astronomy is just pretty pictures. I like pretty pictures, but it can be hard to learn from them! So this week's Snack is more physics than astronomy, though the two subjects overlap. But don't worry, it won't be heavy. In fact, it's light as can be!

How many times have you heard the phrases "escape from Earth's gravity" and "zero gravity"? Well, if you want to get picky, the phrases are both wrong. Gravity goes on forever, and there is no place in the Universe where there isn't some gravitational force.

Anything with mass has gravity. Gravity is the force you feel pulling you toward that mass, and the amount of the force depends on two things: the amount of mass, and the distance you are from that mass. If you double the mass, you double the force. If you double the distance, though, the force drops by a factor of four; the square of the distance change. If you go ten times farther away, the force drops by 10 x 10 or 100 times.

Now think about this: you've probably heard journalists talk about the astronauts aboard Mir or the Space Shuttle being in zero gravity. But we know the Moon is held to the Earth by gravity, yet the Moon is typically 40 or so times farther from the Earth than the astronauts are (for gravity, you measure from the actual center of the Earth). So if the Earth's gravity is strong enough to hold the Moon in orbit, how can the astronauts be in zero gravity?

It's because they aren't. The gravity is stronger where the astronauts are than at the Moon! They are weightless, but there is gravity there. How can that be? Well, what we think of as weight is actually two things: it's gravity pulling us down, and the force of the ground pushing us up. We feel our feet supported by the ground, and we call that feeling weight. But if the ground were to drop away, we would have nothing pushing us back up. We'd feel weightless, yet gravity would still act on us, pulling us down. This is what happens when you go down in an elevator. The floors drops a bit, letting gravity have its way with you, and you feel lighter for a moment. That's also what happens with astronauts: gravity pulls them down, but they move sideways so quickly that they basically keep missing the Earth. They are falling freely (freefall, get it?), and so they feel no weight, but there is still gravity pulling them. Note too that the force of gravity is not perfectly towards the center of the Earth for them, because the Earth is not a perfect sphere, and because of lots of other effects. For that reason, NASA calls this condition "microgravity", not zero gravity.

One more thing. Since gravity drops with distance squared, it never really gets to zero, no matter how far you are from a mass. You could go clear across the Universe and the Earth's gravity will still be there. So there really is no place in the Universe where there is zero gravity, except in the annals of Bad Astronomy.

©2008 Phil Plait. All Rights Reserved.

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