Gravitation: The force of attraction of two or more obects by an objects mass. The mass is determined by the amount of matter in the object.
In Einstein 's theory of relativity, he theorized that matter and energy change the shape of space-time, and the distortion is experienced as gravity.
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Primack, Joel R. "Gravitation." World Book Online Reference Center. 2004. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar233440.