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.

What do we mean by the mass of a body? It is not easy to define certain fundamental quantities like “mass”. One way of defining it is on the basis of the fact that the mass of an object is the measure of its inertia. Such a mass is known as “**inertial mass**”. If a force “F” acting on a body produces an acceleration “a” in the body, then, its inertial mass is defined as the ratio of “F” and “a”.

The SI unit for mass is the “kilogram”. When a body is placed in the earth’s gravitational field, the body is attracted by the earth. The force with which the earth attracts a body is known as the “**weight of the body**”.

If the acceleration gained by a body due to the earth’s gravitational attraction is “g,” then its weight is equal to “mg”.

Weight of a body = mg

Since the “weight” of an object is a force, its SI unit is the “newton”.

The weight of a body is a vector quantity, and always acts towards the centre of the earth. If a body is taken from the “earth” to the “moon”, then there will be no change in its mass, but its weight will decrease. This is because the moon attracts the body with less force than that exerted by the earth. In fact, the **weight of a body** on the surface of the **moon** is only **one-sixth of its weight** on the surface of the earth.