![]() ![]() Torsion balance experiments to measure G consisted of a 'dumbbell' (two masses connected by a horizontal rod) suspended by a very thin fiber. ![]() Most measurements of G have used variations of the torsion balance technique first used by Cavendish. Naturally, the value of the fundamental constant G has interested physicists for over 300 years and, except for the speed of light, it has the longest history of measurements. Early in this century Albert Einstein developed his theory of gravity called General Relativity in which the gravitational attraction is explained as a result of the curvature of space-time. When asked why he was measuring G, Cavendish replied that he was "weighing the Earth" once G is known the mass of the Earth can be obtained from the 9.8 m/s 2 gravitational acceleration on the Earth surface and the Sun's mass can be obtained from the size and period of the Earth orbit around the sun. However, more than 100 years elapsed before G was first measured in the laboratory in 1798 Cavendish and co-workers obtained a value accurate to about 1%. Newton estimated this constant of proportionality, often called Big G, perhaps from the gravitational acceleration of the falling apple and an inspired guess for the average density of the Earth. In 1686 Isaac Newton realized that the motion of the planets and the moon as well as that of a falling apple could be explained by his Law of Universal Gravitation, which states that any two objects attract each other with a force equal to the product of their masses divided by the square of their separation times a constant of proportionality. The story of the gravitational constant, Big G:
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