gravity

The gravimeter is an instrument for measuring the force of gravity at a particular location. Variations in the force of gravity acting on a weight suspended by a spring cause the spring to stretch. The gravimeter is used in aerial surveys. Geological features such as intrusions and salt domes are revealed by the stretching of the spring.

The gravitational pull of the Moon is the main cause of the tides. Water on the side of the Earth nearest the Moon feels the Moon's pull and accumulates directly under the Moon. When the Sun and the Moon are in line, at new and full Moon, the gravitational pull of Sun and Moon are in line and produce a high spring tide. When the Sun and Moon are at right angles, lower neap tides occur.

A portrait of the English physicist and mathematician Isaac Newton, painted before the end of the 17th century. From 1665 to 1666, when he was 23, Newton stayed at home to avoid an outbreak of plague; during that year he is believed to have formulated many of his most important ideas.

Force of attraction that arises between objects by virtue of their masses. The larger the mass of an object the more strongly it attracts other objects. On Earth, gravity causes objects to have weight; it accelerates objects (at 9.806 metres per second per second/32.174 ft per second per second) towards the centre of the Earth.

The Earth's gravity also attracts the Moon towards the Earth, keeping the Moon in orbit around the Earth. The Moon's gravity is one-sixth that of Earth's, so objects on the Moon weigh less than on Earth. The Sun contains 99.8% of the mass of the Solar System, and the resulting large force of gravity keeps the planets of the Solar System in orbit around the Sun.

A spacecraft launched from Earth must overcome the force of gravity before entering space. This is achieved by using rocket boosters at various stages of the launch. The spacecraft needs an acceleration of three times that of gravity (3 g). The speed required to escape the Earth's gravitational field is 40,000 kph/25,000 mph.

Fundamental force of nature Gravity 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.

Measuring gravity 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 masses was first planned in the mid-18th century by the Reverend J Michell (1724–1793), who did not live to work on the apparatus he had designed and completed. After Michell's death the apparatus came into the hands of Henry Cavendish, who largely reconstructed it but kept to Michell'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 light 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 6.6742 × 10−11 N m2 kg−2.

Newton's laws 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 centre 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 approximately 9.806 metres per second per second/32.174 feet per second per second (9.806 m s−2/32.174 ft s−2).

Escape velocity This is the velocity that a projectile or spacecraft needs to escape from a gravitational field. The escape velocity from the surface of the Earth is about 40,000 kph/25,000 mph; that from the Moon (with one-sixth the gravitational pull of the Earth) is about 8,500 kph/5,300 mph.

Relativity Albert Einstein's general theory of relativity treats gravitation 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 pulsars (which emit energy at regular intervals) in orbit around other bodies have shown that the objects are spiralling together at the rate that would be expected if they were losing energy in the form of gravitational waves.