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Gravity Definition Page - Windows to the Universe

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With Explore the Planets, investigate the planets, their moons, and understand the processes that shape them. By G. Jeffrey Taylor, Ph.D. See our DVD collection.
An artist's rendering of some of the forces of the universe. The apple falling is of course from the story of Isaac Newton discovering the law of gravity as an apple fell from a tree he was sitting underneath.
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Gravity Definition Page

Gravity is one of the universal forces of nature. It is an attractive force between all things. The gravitational force between two objects depends on their masses, which is why we can really only see gravity in action when at least one of the objects is very large (like the Earth).

Isaac Newton was the first scientist to define gravity using math. There will be a larger pull due to gravity if the objects' masses are larger and if the objects are closer together.

One thing we can do with Newtonís law is calculate an escape velocity for the Earth. That is the speed something has to go for it to escape the gravity of Earth. This number for Earth is about 11 km/s. This means that if you could throw a baseball at 11 km/s, it would never come down!

Last modified May 6, 2008 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF