Gravity

An object released from some high above the surface of the earth falls freely with an acceleration. This accelerate motion is due to the force of attraction exerted by the earth on the object. The motion of the moon in a circular orbit around the earth also shows that the earth exerts a force of attraction between the sun and suggest the existence of a force of attraction between the sun and the plant. These observations led Newton  to the conclusion that any two material objects always attract each other. This attraction is called gravity or gravitation and the force of attraction is called the gravitational force.
Newton’s law of gravity or gravitation:
The gravitational force between any two material object is given by Newton’s law of gravitation, which is every particle of matter attraction every other particle of matter with a force which is directly proportional to the product of their masses and inversely proportional to the square of distance between them.
Suppose two particles of masses m₁ to m₂ separated by a distance r them according to Newton’s law of gravitation, these particles attract each other with a force whose magnitude (F) is given by
F::m1m2 / r2
f =
where  G is a constant called constant of gravity.
SI unit of GI is Nm²/kg² its dimensions can be determined
[G] =
=
=  [M ^-1L³ T^-2]
The gravitational force between two particles act along the line joining the two particles and they form an action reaction pair. The force exerted by the first particle on the second particle is exactly equal and opposite to the force exerted by the second particle on the first.
Newton’s law of gravitation holds good for all material object irrespective of their sizes or distance between them. Therefore it is called a universal law and the constant of gravity G1 is called the universal constant its value is 6.673 x 10^-11 Nm²/kg². In order to explain how to masses attract each other even though there is no physical contact between them, the concept of gravitational field is introduced. According to this concept, there exists a gravitational field in the space surrounding and mass. When another mass is brought into this space, it is acted upon by the gravitational force of attraction.