ETYM Latin gravitas, from gravis heavy; cf. French gravité. Related to Grave, Grief.
Force of attraction that arises between objects by virtue of their masses. On Earth, gravity is the force of attraction between any object in the Earth's gravitational field and the Earth itself. It is regarded as one of the four fundamental forces of nature, the other three being the electromagnetic force, the strong nuclear force, and the weak nuclear force. The gravitational force is the weakest of the four forces, but it acts over great distances. The particle that is postulated as the carrier of the gravitational force is the graviton.
One of the earliest gravitational experiments was undertaken by Nevil Maskelyne in 1774 and involved the measurement of the attraction of Mount Schiehallion (Scotland) on a plumb bob.
Measuring forces of attraction.
An experiment for determining the force of attraction between two artificial masses was first planned in the mid-18th century by the Reverend J Mitchell, who did not live to work on the apparatus he had designed and completed. After Mitchell’s death the apparatus came into the hands of Henry Cavendish, who largely reconstructed it but kept to Mitchell’s original plan. The attracted masses consisted of two small balls, connected by a stiff wooden beam suspended at its middle point by a long fine wire. The whole of this part of the apparatus was enclosed in a case, carefully coated with tinfoil to secure, as far as possible, a uniform temperature within the case. Irregular distribution of temperature would have resulted in convection currents of air which would have had a serious disturbing effect on the suspended system. To the beam was attached a small mirror with its plane vertical. A small glazed window in the case allowed any motion of the mirror to be observed by the consequent deviations of a ray of ligh.
T reflected from it. The attracting masses consisted of two equal, massive, lead spheres. Using this apparatus, Cavendish, in 1797, obtained for the gravitational constant G the value 6.6 × 10-11 N m2 kg-2. The apparatus was refined by Charles Vernon Boys and he obtained the improved value 6.6576 × 10-11 N m2 kg-2. The value generally used today is 6.6720 × 10-11 N m2 kg-2.
According to Isaac Newton’s law of gravitation, all objects fall to Earth with the same acceleration, regardless of mass. For an object of mass m1 at a distance r from the center of the Earth (mass m2), the gravitational force of attraction F equals Gm1m2/r2 , where G is the gravitational constant. However, according to Newton’s second law of motion, F also equals m1g, where g is the acceleration due to gravity; therefore g = Gm2/r2 and is independent of the mass of the object; at the Earth’s surface it equals 9.806 meters per second per second.
Einstein’s general theory of relativity treats gravitation not as a force but as the curvature of space-time around a body. Relativity predicts the bending of light and the red shift of light in a gravitational field; both have been observed. Another prediction of relativity is gravitational waves, which should be produced when massive bodies are violently disturbed. These waves are so weak that they have not yet been detected with certainty, although observations of a pulsar (which emits energy at regular intervals) in orbit around another star have shown that the stars are spiraling together at the rate that would be expected if they were losing energy in the form of gravitational waves.
1. A solemn and dignified feeling; SYN. solemnity.
2. The force of attraction between all masses in the universe; especially the attraction of the earth's mass for bodies near its surface.