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Q Why is there gravity on Earth but not in space?

— Brayden Banyai, Madison, Wis.

A Jim Lattis, director of the UW Space Place at the University of Wisconsin-Madison:

There’s actually gravity pretty much everywhere. But why do we feel gravity more here on the surface of the Earth instead of in space when astronauts appear weightless?

Those astronauts are very much attracted by the gravity of the Earth, but they’re not feeling it because they’re in what we call free fall.

Think of a skydiver. Skydivers in an airplane feel the weight of gravity before jumping out of the plane. Then they jump, and before they open their parachute, they are falling freely. The skydiver won’t feel any gravity at that point until opening a parachute.

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If we put the skydiver into space — let’s say halfway between the Earth and the moon — the same thing will happen. The skydiver will start falling toward the Earth but won’t feel it.

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If you’re in free fall, you don’t feel the gravity because everything around you is falling with you. It’s an unusual feeling to imagine and something unfamiliar for us on Earth, unless you’re a skydiver or an astronaut.

You can experience it a bit when an elevator starts to move downward and you feel a slight lurch. That’s a little lessening of the force of gravity that you’re feeling on the floor of the elevator.

The moon and other objects in orbit are doing exactly the same thing. The moon is in free fall, and it’s being attracted by the force of the Earth’s gravity. It would fall straight into the Earth, but it’s moving perpendicular to that line of force fast enough to stay in orbit.

If you were standing on the surface of the moon, you would feel the gravitational attraction of the moon. The gravitational force on the moon is less than on Earth because the strength of gravity is determined by an object’s mass. The bigger the object, the bigger the gravitational force.

Gravity is pretty much everywhere. We just feel it in different ways depending on our state of motion.

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Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.

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