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NASA
Nasa: Newton's Theory of "Universal Gravitation"
This site, which is provided for by NASA, discusses the thinking which led Newton to understand universal gravitation.
Physics Classroom
The Physics Classroom: Circular and Satellite Motion: The Inverse Square Law
Through illustrated examples and interactive practice, students explore Newton's ability to relate the cause for heavenly motion (the orbit of the moon about the earth) to the cause for Earthly motion (the falling of an apple to the...