Article updated: 1/24/2012 11:55 AM

Kids want to know: What causes gravity?

 Young visitors experiment with the effects of gravity on the moon, Earth and Jupiter in a hands-on interactive in the Gravity exhibition at the Adler Planetarium in Chicago.   Courtesy of Adler Planetarium
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By Hope Babowice

You wanted to know

Students in Elise Diaz's fifth-grade class at O'Plaine School in Gurnee asked, "What causes gravity?"

The Warren Newport Library in Gurnee suggests these titles on gravity:

Ÿ "Gravity is a Mystery" by Franklin M. Branley

Ÿ "I Fall Down" by Vicki Cobb

Ÿ "Friction and Gravity: Snowboarding Science" by Marcus Figorito

Ÿ "Falling for Fun: Gravity in Action" by Nathan Lepora

Ÿ "Gravity" by Melinda Lilly

What exactly is gravity?

We know gravity exists because of what doesn't happen. When we wake up we're still in our beds, not floating near the bedroom ceiling. When we brush our teeth there's no mad dash to grab the toothbrush as it sails around the bathroom.

Scientists have been asking that question for thousands of years and still are working on an explanation. Gravity is the force that gives weight to objects and causes every particle to be attracted toward every particle.

The size and distance between the two items determines how much gravity is involved.

"Gravity is a force that acts between any two objects, no matter how far apart or how small," said Geza Gyuk, director of astronomy at Chicago's Adler Planetarium.

"The penny in my pocket is exerting a force on the umbrella I lost in Rome 15 years ago, and the drop of dew on a blade of grass is pulling ever so slightly on the swirling disk of gas surrounding a distant star."

Here are the basic ideas that go into defining gravity. The more massive the object the greater the force. The greater the distance between objects, the smaller the gravitational force.

"Every time you double the distance, you make the force four times smaller," Gyuk said. "If the distance is increased by a factor of three, then the force is decreased by a factor of nine."

Galileo Galilei, the 17th-century astronomer, figured out that air resistance is what makes things fall faster or slower, not gravity.

"The force of gravity doesn't have anything to do with what the objects are made of," Gyuk said. "A feather and a steel anvil fall at the same rate if you can ignore air resistance."

In the 1900s, Albert Einstein added a new curve to the gravity concept, literally.

"Einstein's theory says gravity is a bending of space and time," Gyuk said. "It is very difficult to understand this or visualize it, but this treatment of gravity, called General Relativity, makes very precise predictions about gravity both under normal circumstances and also in extreme cases when describing huge masses and incredible speeds close to that of the speed of light."

In everyday life, gravity keeps our feet on the ground when we're walking, causes the tides to move back and forth, and holds the moon, planets and space matter in orbit.

Learn more about gravity at the Adler Planetarium exhibit "Gravity Shapes the Universe." See adlernet.org for museum times and admission.

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