Gravitation :



I INTRODUCTION
Gravitation, property of mutual attraction possessed by all bodies. The term “gravity” is sometimes used synonymously. Gravitation is one of four basic forces controlling the interactions of matter; the others are the strong and weak nuclear forces and the electromagnetic force (seePhysics). Attempts to unite these forces in one grand unification theory have not yet been successful (seeUnified Field Theory), nor have attempts to detect the gravitational waves that relativity theory suggests might be observed when the gravitational field of some very massive object in the universe is perturbed.
The law of gravitation, first formulated by Isaac Newton in 1684, states that the gravitational attraction between two bodies is directly proportional to the product of the masses of the two bodies and inversely proportional to the square of the distance between them. In algebraic form the law is statedwhere F is the gravitational force, m1 and m2 the masses of the two bodies, d the distance between the bodies, and G the gravitational constant.

The value of this constant was first measured by the British physicist Henry Cavendish in 1798 by means of the torsion balance. The best modern value for this constant is 6.67 × 10-11 N m2 kg-2. The force of gravitation between two spherical bodies, each with a mass of 1 kilogram and with a distance of 1 metre between their centres, is therefore 6.67 × 10-11 newtons. This is a very small force; it is equal to the weight (at the Earth’s surface) of an object with a mass of about 0.007 micrograms (a microgram is one millionth of a gram).


II EFFECT OF ROTATION
The measured force of gravity on an object is not the same at all locations on the surface of the Earth, principally because the Earth is rotating. The measured, or apparent, weight of the object is the force with which the object presses down on, for example, the pan of a spring scale. This is equal to the reaction force with which the pan presses upward on the object. Any object travelling at constant speed in a circle is constantly accelerating towards the centre of the circle (seeMechanics: Kinetics). This centre-directed acceleration has to be sustained by a centre-directed force, or centripetal force. In the case of the object being weighed at the Earth’s surface, the centripetal force is the result of the fact that the upward supporting force from the pan of the spring balance is slightly less than the object’s weight.


III ACCELERATION
Gravity is commonly measured in terms of the amount of acceleration that the force gives to an object on the Earth. At the equator the acceleration of gravity is 977.99 cm s-2 (centimetres per second per second) (32 9/100 ft s-2 ) and at the poles it is more than 983 cm s-2. The generally accepted international value for the acceleration of gravity used in calculations is 980.665 cm s-2 (32 1/6 ft s-2). Thus, neglecting air resistance, any body falling freely will increase its speed at the rate of 980.665 cm s-1 (32 1/6 ft s-1) during each second of its fall. The apparent absence of gravitational attraction during space flight is known as zero gravity or microgravity.