Between any two objects of masses m1 and m2 at separation r from each other there exist attractive forces 
and 
directed from one body to the other and equal in magnitude
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Which is instantly proportional to the product of the masses of the bodies and inversely proportional to the square of the distance between the two,
In vector form  .
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Where G is universal gravitational constant. G = 6.67 ´10 -11 Nm2 / kg2
Since G is very small, so its effect cannot be experienced in daily use. However in the case of celestial bodies which have very large masses, the forces of attraction are very large, In fact, in their case, this force provides the necessary centripetal force to keep them in specific orbits.
Dimensional formula of G: 
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Vector form : Given by Newton's law of gravitation
Here minus sign shows that the direction of  is opposite to that of  .
Similarly 
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 = unit vector from A to B
= unit vector from B to A,
= gravitational force operated on body A by body B
 = gravitational force operated on body B by body A It is clear that . This is Newton's third law of motion.
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Here G is fixed value of proportionality which is called 'Universal gravitational constant'.
If m1 = m2 and r = 1 then G = F
i.e. universal gravitational constant is equal to the force of attraction between two bodies each of unit mass whose centers are placed unit distance apart.
But if the two objects are uniform spheres then the separation r may be taken as the distance between their centers because a sphere of uniform mass behave as a point mass for any point lying outside it.
Gravitational Lines of forces:
Gravitational field can also be represented by lines of force. This is same in various ways as that of electric lines of forces. With single dissimilarity, electric charges are negative and positive which causes force of attraction between dislike charges and repulsion between like charges. While in phase of gravitation the force is always of attraction.
A line of force is drawn in such a way that at each point the direction of field is tangent to line that passes through the point. Thus tangent to any point on a line of force gives the direction of gravitational field at that position. Lines of forces are given in such a way that their density is proportional to the strength of field.
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