Teaching Physics with the Physics Suite

Edward F. Redish

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Is Newton's law of gravity wrong?

A professional scientist (not a physicist) once stopped me in the hall and said: "I can prove Newton's theory of gravity is wrong. The sun is 320,000 times as massive as the earth, but only 400 times as far from the moon as the earth is. Therefore the force of the sun's gravity on the moon should be twice as big as the earth's and the moon should go around the sun instead of around the earth. Since it doesn't, Newton's theory of gravity must be wrong!" What's the matter with this reasoning?

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Page last modified October 15, 2002: GR04