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Every object in the universe attracts the other with a force. This is by virtue of the mass of the objects. This force of attraction was supposed to be thought upon by Newton while contemplating on the free fall of an apple towards the ground. The force of attraction, which is the gravitational pull due to mass of objects, exists universally. The factors that affect gravitational force were studied and a law was put forth which is known as 'Newton's Universal Law of Gravitation'. The gravitational force between two objects in the universe is directly proportional to the mass of the objects and is inversely proportional to the square of the distance between them. Hence, the mathematical form of the law is where 'm1', 'm2' are the masses of the objects and 'r' is the distance between them. Equating both sides of the expression we get, where, G is the constant of proportionality called the 'universal gravitational constant'.
The second part of the law is called the 'inverse square rule' or 'inverse square law'. The force with which earth attracts any object on its surface is the weight (W) of the object, which is the product of the mass (m) of the object and its acceleration due to gravity (g). 'W' changes from place to place on the earth on account of variation in 'g'. Thus the mass of an object remains the same throughout the universe where the weight of an object changes from place to place.