Today, the currently accepted value is 6.67259 x 10 -11 N m 2/kg 2. Cavendish's measurements resulted in an experimentally determined value of 6.75 x 10 -11 N m 2/kg 2. By measuring m 1, m 2, d and F grav, the value of G could be determined. Once the torsional force balanced the gravitational force, the rod and spheres came to rest and Cavendish was able to determine the gravitational force of attraction between the masses. Since all masses attract, the large spheres exerted a gravitational force upon the smaller spheres and twisted the rod a measurable amount. A diagram of the apparatus is shown below.Ĭavendish then brought two large lead spheres near the smaller spheres attached to the rod. ii despite its name, big g is tiny about 6.67 x 10. Cavendish had calibrated his instrument to determine the relationship between the angle of rotation and the amount of torsional force. the gravitational constant is familiarly known as 'big g' to distinguish it from 'little g,' the acceleration due to the earth's gravity. The more twist of the wire, the more the system pushes backwards to restore itself towards the original position. When the rod becomes twisted, the torsion of the wire begins to exert a torsional force that is proportional to the angle of rotation of the rod. Two small lead spheres were attached to the ends of the rod and the rod was suspended by a thin wire. The value of G was not experimentally determined until nearly a century later (1798) by Lord Henry Cavendish using a torsion balance.Ĭavendish's apparatus for experimentally determining the value of G involved a light, rigid rod about 2-feet long. The constant of proportionality in this equation is G - the universal gravitation constant. In equation form, this is often expressed as follows: Isaac Newton's law of universal gravitation proposed that the gravitational attraction between any two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
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