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Haplosciences.com |
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Newton’s law of universal gravitation Motion in the heavens |
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Newton proposed his law of universal gravitation F = G m M/r2 But he didn’t know the value of G/ 100 years later Henry Cavendish (1731-1810) shows that the formula was right and was able to compute G. Once he got G he was able to find M the mass of the Earth. Cavendish is know as the scientists who weighted the Earth. Cavendish Found the value of G in the formula F = G m M/r2 If F is the force between you and the Earth, m is your mass, M is the mass of the Earth and F is your weight. You know the acceleration due to gravity on Earth (g) and if you know g, you can easily find M.
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He attached a lead ball to each end of a long rod and suspended the rod by a thin wire. Then he carefully measured the amount of force required to rotate the wire through given angles. Next he places two large lead balls close to small ones as shown. The two small balls were attracted by the two large balls. This caused the wire to twist. By measuring the angle through which the wire turned. Cavendish was able to find the force between the lead masses. He found that the force exactly followed the law of gravitation.
Cavendish measured the masses of the balls and the distance between their centers. Substituting these values for force, mass, and distance into the law of gravitation, he determined the value of G for Newton’s law of universal gravitation. G= 6.67 10-11 N. m2/kg2 |
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