# Gravity

###### Priyanka Devan
Updated on

Gravity is also called gravitation. The universal force of attraction acting between all matter. It is the weakest known force in nature and also thus plays no role in determining the internal properties of matter. It has long reach and universal action and it controls the trajectories of bodies in the solar system and elsewhere in the universe and the structures. On Earth all bodies have a downward force of gravity, it is proportional to their mass. It is measured by the acceleration that it gives to freely falling objects. At earth’s surface the acceleration of gravity is about 9.8 metres per second per second.  For every second an object is in free fall  its speed increases by about 9.8 metres per second. At the surface of the moon the acceleration of a freely falling body is about 1.6 metres per second per second.

## Newtons law of gravity

Newton discovered the relationship between the motion of the moon, and the motion of a body falling freely on earth. By his gravitational theories, he explained Kepler’s laws. He established the modern quantitative theory of gravitation. He assumed the existence of an attractive force, between all massive bodies. By his law of inertia, he concluded that a force exerted by Earth on the Moon is needed to keep it in a circular motion. When Newton discovered that the Moon is 1/3,600 smaller than the acceleration at the surface of Earth.  He related the number 3,600 to the square of the radius of Earth. He also calculated that the circular orbital motion of radius R and period T ,

The Moon has a radius of about 384,000 km , and its period is 27.3 days , or the period measured in terms of lunar phases. Newton found the Moon’s inward acceleration in its orbit to be 0.0027 meter per second per second, the same as (1/60)2 of the acceleration of a falling object at the surface of Earth.

In Newton’s theory the particle of matter attracts every other particle gravitationally. More generally, the attraction of any body at a sufficiently great distance is equal to that of the whole mass at the centre of mass. He  related the two accelerations, a gravitational force between bodies that diminishes . If the distance between the bodies is doubled, the force on them is reduced to a fourth of the original.

## Weight and mass

The equal and opposite force necessary to prevent the downward acceleration helps to measure the weight of the body. Since the Moon has a mass of about 1/81 times that of Earth and a radius of just 0.27 that of Earth, the body on the lunar surface has a weight of only 1/6 its Earth weight . But the same body  when placed on the surface of the Moon has the same mass. In orbiting satellites passengers and instruments  are in free fall. Even though their masses remain the same as on Earth they experience weightless conditions.

## Changes with time

The gravitational potential at the surface of Earth is mainly  due to the mass and rotation of Earth.  There are also small contributions from the distant Sun and Moon. Those small contributions at any one place vary with time, and so the local value of g varies slightly as Earth rotates. They are called the diurnal and semidiurnal tidal variations. For most purposes it is necessary to know only the changes of gravity from place to place  or the variation of gravity with time at a fixed place. Then  the tidal variation can be removed.  Almost all gravity measurements are relative measurements of the differences  from time to time or from place to place .

## Field theories of gravitation

Physicists Abdus Salam of Pakistan and Steven Weinberg and Sheldon L. Glashow of the United States was able to show that the weak force and electromagnetic forces  responsible for beta decay were different manifestations of the same basic interaction. This was the first successful unified field theory. During the 1970s and ’80s the possibility that gravitation might be linked with the other forces of nature in a unified theory of forces greatly increased interest in gravitational field theories. Because the gravitational force seems to be independent of all physical properties except mass  and since it is exceedingly weak compared with all others the unification of gravitation with the other forces remains the most difficult to achieve. This challenge has provided a tremendous impetus to determine whether there may be some failure of the apparent independence.